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. This question can be resolved by the dependence of the writing rate on spot size. Measurements of the process show that once the deposition begins the bulk of the deposit occurs from decomposition in the gas phase.