Experiments were performed to determine the heat transfer, pressure drop, and flow field responses to the rounding of the peaks of a corrugated-wall duct. Two different degrees of corrugation-peak roundedness were used in addition to sharp (i.e. unrounded) corrugation peaks. The experiments encompassed the Reynolds number range from 2000 to 33 000 while the Prandtl number ranged from 4 to 11. It was found that at a given Reynolds number (i.e. given mass flow rate), the rounding of the corrugation peaks brought about a decrease in the Nusselt number which was accentuated at larger Reynolds numbers. The friction factor corresponding to a given Reynolds number decreased even more than did the Nusselt number. On the other hand, at equal pumping power, the Nusselt number was relatively insensitive to whether the peaks were sharp or rounded. Flow visualization experiments showed that rounding reduces the size of the separated region that is spawned at each corrugation peak.