After 10 minutes of searching I can't find it, found this:
CruelDM
Member Join Date: Oct 2004
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Quote:
Originally Posted by 3.5 PHB pg. 124
Buckler: This small shield is worn strapped to your forearm. You can use a bow or crossbow without penalty while carrying it. You can also use your shield arm to wield a weapon (whether you are using an off-hand weapon or using your off-hand to help wield a two-handed weapon), but you take a -1 penalty on attack rolls while doing so because of the extra weight on your arm. This penalty stacks with those that may apply for fighting with your off-hand or fighting with two weapons. In any case, if you use a weapon in your off-hand, you dont get the bucklers AC bonus for the rest of the round.
So, as we can all clearly see, by the RAW, you can use a buckler when wielding a two-handed weapon, but if you do you suffer a -1 to attack with any weapon in your off-hand (including a two-handed weapon other than a bow) and lose the AC benefit for the rest of the round.
Improved Buckler Defense feat allows you to retain the AC benefit even if you use the off-hand to attack (yes even when attacking with a two-handed weapon).