We have compared with good precision between 1.5 and 20 K the specific heat of two well characterized copper samples with residual resistivity ratioR(293 K)/R(4 K) of approximately 2000 (Calorimetry Conference, Argonne National Laboratories) and 30 000 (National Bureau of Standards, Boulder), and find that the electronic specific heat of the former is about 1% higher in the range 1.5–5 K, although the Debye temperatures are the same. The total specific heat of the former is up to 0.8% higher throughout the 1.5–20 K range. This discrepancy indicates that the specific heat may depend on the residual resistivity ratio. The germanium thermometer used in the measurements was calibrated against the National Bureau of Standards acoustic temperature scaleT65, and so a test has been made of the smoothness of that scale.