Experimental results for the deposition of low dielectric constant Parylene-N and Parylene-F films are explained by a mechanism based on bulk phase diffusion reaction in the firn. The deposition rate of Parylene-F decreases significantly as time increases and ultimately achieves steady state, Very favorable agreement with experiments at a substrate temperature of -10 degrees C is obtained for parylene-F with a rate constant of k(f)=5X10(-5)/s, and a diffusion coefficient D-f=1.3X10(-8) cm(2)/s. In contrast, the much higher rate constant at room temperature for Parylene-N films is k(f)=0.16/s and the predicted deposition rate is constant after a very small period of time (t(ss)<4/k(f)=25 s). This prediction is confirmed by the experiments. The model predicts correctly that low deposition temperatures improve both the deposition rate and thermal stability of Parylene films, Our experiments confirm this prediction and one obtains both high growth rates and high molecular weight films at lower deposition temperatures, Furthermore, experiments show that the longer chains formed at low temperatures significantly improve the thermal stability of the films, The deposition rate also can be improved by using higher reactor pressures, However, the pressure that can be employed is limited to about 100 mTorr to maintain high quality films, At liquid nitrogen temperature (77 K), the deposition rate is three orders of magnitude larger than the film deposition rate at room temperature. The increased monomer concentration on the substrate due to higher condensation leads to this increased deposition rate.