Measurements have been made of the effective thermal conductivity of porous sandstones. The method is based on the transient heating effect resulting from use of a line heat source. Data are presented for six sandstones ranging in porosity from 3 to 59% and show the variation of thermal conductivity with porosity, the conductivity of the saturating fluid, the pressure of the gas filling the pore space, and overburden pressure. The results are compared with those previously obtained for unconsolidated sands. All samples, except one, exhibited a lower thermal conductivity when saturated with a gas at atmospheric pressure than when saturated with a liquid of the same conductivity as the gas. An explanation for this effect, in terms of the kinetic theory of gases, is advanced and substantiated by other data. Finally, the validity of certain equations for the thermal conductivity of two‐phase systems is examined; the weighted geometric mean of the two constituent conductivities is found to agree well with the measured effective conductivities.