Direct contact heat transfer between water and a heat transfer oil was investigated under non-boiling conditions in co-current turbulent flow through a horizontal concentric annulus. The ratio of the inner pipe diameter to the outer pipe diameter (aspect ratio) kappa = 0.730-0.816; total liquid velocity (mixture velocity) V-T = 0.42-1.1 m/s; inlet oil temperature T-ci = 38-94 degrees C; oil volume fraction in the flowing mixture phi(0) = 0.25-0.75 were varied and their effects on the overall volumetric heat transfer coefficient U-v were determined at constant interfacial tension of 48 dynes/cm. It was found that, in each concentric pipe set, the overall volumetric heat transfer coefficient increased with increasing dispersed phase volume fraction at each constant mixture velocity and reached a maximum at around phi(0) = phi(W) approximate to 0.5. The maximum U-v values increased with increasing total liquid velocity and decreasing aspect ratio of the annulus. The volumetric heat transfer coefficient was also found to increase with increasing inlet oil temperature and increasing total liquid velocity but to decrease with length along the test section keeping all other parameters constant. Empirical expressions for the volumetric heat transfer coefficient were obtained within the ranges of the experimental parameters.