Photodecomposition of organometallic gases has been shown to be potentially useful for high resolution direct metal deposition. However, a number of problems, particularly the low writing rates, must first be solved. An understanding of the deposition mechanism is essential to this task. In particular, the decomposition must be shown to occur either in the gas phase or in the adsorbed layer. We show that this question can be resolved by the dependence of the writing rate on spot size. Measurements of the process show that the decomposition occurs in the gas phase. This result is shown to cause inhomogeneous deposition for exposures through masks with varying feature sizes, and effect will be important to future applications.