Fallout – How is XP awarded for combat

Each enemy has an assigned experience value. Every instance of the enemy you kill awards the same experience. Any experience modifiers, such as Fast Learner or Skilled, are applied afterwards multiplicatively.

For example, say you have a 10% bonus to exp, and a -10% penalty to exp, and you kill an enemy that gives you 100 exp points. You will always get 99 exp from that kill, regardless of it you increase by 10% then decrease by 10% of the new value or decrease by 10% then increase by 10% of the new value.