Subjecting illuminated n-type silicon wafers to anodic bias in an HF containing electrolyte results in the formation of macropores under certain conditions. In this paper the formation of randomly nucleated macropores is studied as a function of the applied potential, the temperature, and the doping levels of the samples. A large number of micrographs was evaluated by computerized image processing and the data obtained are compared to predictions of pore formation models. It was found that the formation of randomly nucleated macropores involves a prolonged nucleation phase. Starting from a polished surface, first macropores occur after a certain amount of Si has been homogeneously dissolved. In this nucleation phase the thickness of the homogeneously dissolved Si depends strongly on the doping level and the temperature, but only weakly on the applied bias. In a second phase of stable pore growth, the density of pores is investigated as a function of temperature and anodic potential. For low-doped material a strong influence of the space-charge region on the average macropore density is observed in accordance with existing models; an increased anodic bias, e.g., decreases the density of pores. For highly doped silicon the situation reverses; increasing anodic bias increases the pore density, in contrast to predictions. The pore growth in this region is not very sensitive to the space-charge region but seems to be dominated by the chemical-transfer rate.