We show harmonizing domestic GAAP with foreign GAAP can have deleterious effects on security market performance, specifically price informativeness and trading volume. Harmonization effects result from interaction between two forces. Direct informational effects depend on whether harmonization increases or decreases GAAP precision. Expertise acquisition effects depend on benefits and costs to foreign investors of becoming domestic GAAP experts. These countervailing forces can result in harmonization to more (less) precise GAAP increasing (decreasing) or, unexpectedly, decreasing (increasing) price informativeness and trading volume. We also observe this for a cost of capital metric. Thus, harmonization is not necessarily a desirable singular goal.