Polyphenylene oxide (PPO) films have been electrochemically deposited onto gold substrates, yielding ultrathin films on the order of 10 nm. The deposition was monitored by in situ ellipsometry in real time? in order to follow the growth and determine the mechanism. The films were subsequently characterized by ex situ spectroscopic ellipsometry to confirm the final film thickness and growth rate. A combination of ellipsometry and atomic force microscopy observations made in the early and final stages of film formation suggest that a nucleation regime exists. Initially, deposition occurs quickly and is then followed by mass transport controlled growth. A diffusion coefficient of 1.0X10(-15) cm(2)/s was calculated for phenol through a 5 nm PPO film. Two rate constants were determined, one for initial deposition (k(1)=4.7X10(-1) s(-1)) and another for the diffusion controlled regime (k(2)=6.8x10(-5) s(-1)).