Substitution of an electron donor such as phosphorus for a carbon or boron atom in [ital p]-type semiconducting boron carbides is expected to reduce the hole concentration. Phosphorus-doped boron carbide samples have been prepared by hot isostatic pressing with homogeneous dopant levels of up to one phosphorus atom per ten unit cells. Raman spectroscopy and x-ray diffraction confirm the presence of phosphorus within two-atom intericosahedral chains. The high-temperature dc conductivities of doped samples were substantially lower than those of undoped boron carbides. This effect was due to a combination of reduced carrier concentrations and increased hopping activation energies. The low-temperature ac conductivity of doped samples is also smaller than that of undoped samples. However, the number of carriers participating in the ac conduction is a very small fraction ([lt]0.1%) of the total carrier density.