Mixed convection heat transfer in channels with a heated curved surface bounded by a vertical adiabatic wall has been studied numerically. Two cases are considered: in the first case, the flow experiences a convex curvature and an increasing cross-sectional flow area (adverse pressure gradient), while in the second case, the flow experiences a concave curvature with a decreasing flow cross-section (favorable pressure gradient). Results are obtained for various curvature ratios (R/L) of the concave/convex wall, and compared with straight channels of equal heated lengths and equal heights. For channels with a convex heated surface, where the increase in how cross-sectional area decelerates the how while buoyancy accelerates the flow, separation is observed near the heated surface at low values of Gr/Re-2 and on the opposite vertical wall at high values of Gr/Re-2, For low R/L, high Gr/Re-2 and high Pr values, the overall heat transfer on the convex surface is greater than the overall heat transfer in a straight channel of equal height. However, when compared on an equal heated area basis, the convex surface has lower heat transfer. For channels with concave walls, where the decrease in cross-sectional area and buoyancy both accelerate the flow, separation is not observed, and considerably greater heat transfer rates are obtained. The overall heat transfer on the concave surface is always greater than a straight channel of equal height. However, when compared to a straight channel of equal heated surface area, there is a critical Gr/Re-2 value below which heat transfer enhancement is obtained with concave surfaces. (C) 2000 Elsevier Science Ltd. All rights reserved.