When a Pokémon rewards multiple EVs, which one comes first?

If I have a Pokémon that is currently at 509 EVs and I beat an Ivysaur (which grants 1 AtSpe EV and 1 DefSpe EV), which EV will I earn ? (Considering AtSpe and DefSpe aren't yet at 255)

What actual rule decides which one I earn ?

Note : if this mechanism changed, please tell me what it was in the 3rd gen, because that's the gen I'm currently working on.

Solution 1:

In order to answer this question, I've conducted a little experiment using my trusty emulator, a Pokemon Sapphire ROM, and some Gameshark action replay V2 codes. For more information about how to use these codes, read this document over at GameFAQs.

The experiment is set up as follows. Suppose we have a level 99 Mew with a neutral nature and 100 EVs in each statistic except for HP. To get these EVs I used vitamins: Carbos, Calcium, Zinc, and so on, which give 10 EVs in their respective stat up to 100. It looks as follows in the gameboy:

Next, we use the same action replay wild pokemon modifier codes to change the wild pokemon to the following species:

• Nidoran Female, awarding 1 HP
• Machop, awarding 1 ATK
• Muk, awarding 1 ATK and 1 HP

Then we conduct three experiments.

1. Give a rare candy to Mew.
2. Beat 7 machop, 2 nidoran females, and one Muk, then give a rare candy.
3. Beat 2 machop, 7 nidoran females, and one Muk, then give a rare candy.

As you would expect, the results of experiment (1) are that we get a mew with 341 HP and 261 in all other stats. However, experiment (2) and (3) have differing results. See the below two images:

As you can see, before fighting the Muk, the mew would have the following EVs in each experiment:

1. 2 HP, 107 ATK, 100 REST
2. 7 HP, 102 ATK, 100 REST
3. 0 HP, 100 ATK, 100 REST

Now, as we use the stats to infer the information we do not know, and assume that pokemon is coded correctly in that the final EVs do not exceed 510 (which we know to work with pokemon awarding '3' of one single EV but do not know to work with pokemon awarding multiple diffrent EVs), the EVs must be as follows in each experiment:

1. 3 HP, 107 ATK, 100 REST
2. 8 HP, 102 ATK, 100 REST
3. 0 HP, 100 ATK, 100 REST

Since Mew only gained 1 attack over the baseline, the EV can be no higher than 107. It can also be no lower than 107, since 7 machops were fought while Mew's Total EV count was below 510. Hence it is 107.

This leads us to the following conclusion: EV's are applied in a specific order, this means HP evs are applied before attack EVs. The EV's are applied 'one by one' until they can't be applied anymore. More experimentation will confirm what exactly the order is, as these experiments only confirmed HP gets applied before Attack. However, I suspect the order is either:

Version A (presentation order):

1. HP
2. Attack
3. Defense
4. SP Attack
5. SP Defense
6. Speed

or, Version B (order in the game's memory code):

1. HP
2. Attack
3. Defense
4. Speed
5. SP Attack
6. SP Defense

In order to find out which one is correct, we do another experiment, conducted like the last one, in which we fight:

1. 7 vulpix (7 SP. DEF)
2. 2 pidgey (2 SPEED)
3. 1 ninetales (1 SP DEF and 1 SPEED)

The result is this mew:

Here, it's not necessary to do the reverse experiment, as we already proved with the HP/ATK experiment that the final EVs do get applied. (You can talk to the 'rating guy' after fighting the Muk, in both cases he will tell you your pokemon has full EVs).

As you can see, it gained 2 speed over the baseline mew. Thus, the order must be:

1. HP
2. Attack
3. Defense
4. Speed
5. SP Attack
6. SP Defense

You may do further experiments to solidify this claim as per the procedure above (as I only actually proved HP > Attack and Speed > Sp. Def).