Boron-doped microcrystalline (μc) silicon films produced by rf glow discharge from dilute (1%) mixtures of SiH4 in H2 show a critical dependence of conductivity on deposition conditions. The dark conductivity was related to the microscopic features using electron microscopy. The μc−Si:H films contain clusters of crystallites embedded in an amorphous matrix. The size of the crystalline clusters is typically 0.2 μm in diameter, and the size of the individual crystallites is about 2.5 nm. Electron micrographs of samples prepared at substrate temperatures Ts=135°C, 150°C, 165°C, and 180°C show that the number of crystalline clusters increases with Tsup to 165°C. At Ts=180°C, the crystallites completely disappear. When the concentration of SiH4 in H2 is decreased to 0.25%, the microstructure shows a high density of crystallites with no apparent clustering.