Laser holographic interferometry measurements of temperature distributions are presented for a developing rectangular duct with a detached square-rib array and an abrupt-contraction inlet. The Reynolds number based on the duct hydraulic diameter and bulk mean velocity extended from 5.0 x 10(3) to 5.0 x 10(4). The ratios of rib pitch to height, rib to duct height, and detached distance to rib height were 10, 0.13 and 0.58, respectively. Isotherm contours and local and average Nusselt number distributions were used to document the heat transfer characteristics. The complementary pressure loss measurements allowed thermal performance comparison between detached-ribbed, attached-ribbed and smooth ducts to be made. It was found that thermal performance was better in the detached-ribbed duct flows than in the attached-ribbed duct flows. The entry length effect was 5.3-10.6% in terms of length-mean Nusselt number ratio for the detached-ribbed duct flows. Further, semi-empirical correlations for heat transfer and friction in the fully developed region and length mean Nusselt number ratio were developed.