The interphase region of an epoxy/glass fiber model composite was examined by atomic force microscopy phase imaging (AFM-PI) and nanoindentation. The interphase thickness was determined by AFM-PI as a function of gamma-aminopropyl silane coupling agent concentration. With no silane, no measurable interphase was observed. With adsorption from 5 wt% solution, the observed interphase was 888 +/- 30.3 nm thick. Coupling agent adsorption was also performed from 0.1, 1 and 3 wt% silane solutions. The interphase thickness was found to increase with increasing silane solution concentration from 110 to 210 to 375 nm, respectively. Nanoindentation of these same interphases showed that only the 3 wt% and 5 wt% interphases were sufficiently thick enough to not include a significant fiber bias effect. For these two interphases, the indentation depths in the interphase were 8.3% and 42% greater, respectively, than the indentation depth in the matrix.