An experimental study was conducted to determine the lower and upper Reynolds number limits of the transitional flow regime, and the characteristics of the heat transfer coefficients and friction factors for annular passages with different hydraulic diameters and diameter ratios in the transitional flow regime. Water was used in this investigation during isothermal, heating and cooling cases. Four horizontal concentric counter-flow tube-in-tube heat exchangers with conventional inlet geometries were considered to obtain the required data. The flow was both hydrodynamic and thermally developing, and the transitional flow was composed of mixed and forced convection types. The wall temperature on the inner surface of the annular passages was approximately uniform, while the outer surface was isothermal. Average Nusselt numbers were obtained for both the heating and cooling cases, while friction factors were obtained for heating, cooling and isothermal adiabatic conditions. Isothermal adiabatic condition was considered for reference purposes. The geometric size of the annular passage and direction of the heat flux (heating and cooling cases of annular fluid) had a significant influence on the heat transfer coefficients, friction factors and Reynolds number span of the transitional flow regime. The annular geometric parameters that represent the geometric size of the annular passage were proposed and found to describe the heat transfer coefficient and friction factors well. Subsequently, correlations for predicting the Nusselt numbers and friction factors in the transitional flow regime were developed.