Crystallization in a series of variable crosslink density poly(dimethyldiphenyl)siloxanes random block copolymers reinforced through a mixture of precipitated and fumed silica fillers has been studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (I)MA), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). The silicone composite studied was composed of 94.6 mol % dimethoylsiloxane, 5.1 mol % diphenylsiloxane, and 0.3 mol % methyl-vinyl siloxane (which formed crosslinking after peroxide cure). The polymer was filled with a mixture of 21.6 wt % fumed silica and 4.0 wt % precipitated silica previously treated with 6.8 wt % ethoxy-end-blocked siloxane processing aid. Molecular weight between crosslinks and filler-polymer interaction strength were modified by exposure to gamma-irradiation in either air or in vacuo. Isothermal DMA experiments illustrated that crystallization at -85 degrees C occurred over a 1.8 hour period in silica-filled systems and 2.2-2.6 hours in unfilled systems. The crystallization kinetics for irradiated samples were found to be dependent on crosslink density. Irradiation in vacuo resulted in faster overall crystallization rates compared to air irradiation for the same crosslink density, likely due to a reduction in the interaction between the polymer chains and the silica filler surface for samples irradiated in air. (c) 2006 Wiley Periodicals, Inc.