We study the low-temperature electrical and thermal conductivity of CoSi and Co_1−xM_xSi alloys (M = Fe, Ni; x ≤ 0.06). Measurements show that the low-temperature electrical conductivity of Co_1−xFe_xSi alloys decreases at x > 0.01 by an order of magnitude compared with that of pure CoSi. It was expected that both the lattice and electronic contributions to thermal conductivity would decrease in the alloys. However, our experimental results revealed that at temperatures below 20 K, the thermal conductivity of Fe- and Ni-containing alloys is several times larger than that of pure CoSi. We discuss possible mechanisms of the thermal conductivity enhancement. The most probable one is the dominant scattering of phonons by charge carriers. We propose a simple theoretical model that takes into account the complex semimetallic electronic structure of CoSi with nonequivalent charge carrier pockets. This model explains well the increase of the lattice thermal conductivity with increasing disorder and the linear temperature dependence of thermal conductivity in the Co_1−xFe_xSi alloys below 20 K.