The initial‐boundary‐value problem for the transient temperature fields in oil shale, which is heated by a propagating combustion flame in aninsituretort chimney, is formulated and solved analytically. The heat source of the flame is represented by a Gaussian distribution, which models the average statistical variations in radial and axial flame extensions, and is assumed to build up to maximum strength in accordance with a temporal relaxation process. The transient temperature fields in the oil shale surrounding the retort are calculated and discussed with respect to their spatial variations and dependence on the flame relaxation time. The theoretical temperature distributions are shown to be consistent with those observed experimentally. The high‐temperature core extends less than a retort radius into the surrounding shale bed. This result has implications for (i) theinsituretort design and (ii) the environmental impact ofinsituretorts. It appears that the spacing between neighboring retorts can be reduced and that the thermal effects of retorts on the environment are less severe than previously anticipated.