This paper studies the question of when we can eliminate investment inefficiency in a general mechanism design model with transferable utility. We show that when agents make investments only before participating in the mechanism, inefficient investment equilibria cannot be ruled out whenever an allocatively efficient social choice function is implemented. We then allow agents to make investments before and after participating in the mechanism. When ex post investments are possible and an allocatively constrained-efficient social choice function is implemented, efficient investments can be fully implemented in perfect Bayesian Nash equilibria if and only if the social choice function is commitment-proof (a weaker requirement than strategy-proofness). Commitment-proofness ensures the efficiency of investments by suppressing the agents’ incentives to make costly ex ante investments which may work as a commitment device. Our result implies that in the provision of public goods, implementation of efficient investments and efficient allocations is possible even given a budget-balance requirement.