The dewetting of a polycarbonate (PC) film from a poly(styrene-co-acrylonitrile) (SAN) film on a silicon substrate is studied at 180 degrees C, a temperature above the glass transition temperatures of PC and SAN. The viscosity ratio of PC to SAN is similar to 10. Using optical microscopy and video image analysis, the hole diameter D is measured at SAN layer thicknesses L from 125 to 1850 nm. For a PC thickness H of 200 nm, D varies linearly with time, which signifies a constant hole velocity. Initially, the hole velocity increases linearly with L and then becomes nearly constant for L > 1000 nm, which is the dissipation depth. Atomic force microscopy (AFM) studies show that the SAN thickness inside the hole is less than the original L. Analysis of the hydrostatic and dynamic pressures acting on the SAN inside and outside the hole is used to explain this result. For a very thin SAN film (i.e., L similar to 50 nm), D initially increases quickly with time, slows down, and then increases again. Using AFM, the retarded intermediate growth stage is shown to coincide with hole impingement on the silicon substrate. With an increase of H to 310 nm, D is found to accelerate with time. As L decreases from 1850 to 125 nm, the power law exponent ranges from x = 1.1 to 1.6, where D similar to t(x). A cross-sectional transmission electron image of the three-phase boundary shows that SAN is pulled into the rim in contrast to simple dewetting assumptions. Using the measured shape of the rim, the flow field within the rim is calculated as an initial improvement over the simple dewetting model.