This paper presents the performance of R-134a through a diabatic capillary tube with lateral configuration. The paper is divided into two sections: (1) an experimental investigation of straight and helical diabatic capillary tubes and (2) a numerical investigation of the straight diabatic capillary tube. In the experimental investigation, the effects of parameters such as capillary tube diameter, capillary tube length, coil pitch, and inlet subcooling on the refrigerant mass flow rate through a diabatic capillary tube were studied. In addition to the parameters mentioned above, the mass flow rate through a diabatic helical capillary tube was also found to be a function of suction-line inlet superheat and heat exchange length. The suction-line inlet superheat in the present investigation is not a controlled parameter. A comparison of the results of diabatic helical capillary and adiabatic helical capillary tubes was also made. On the basis of acquired experimental data, a correlation to predict the refrigerant mass flow rate was proposed. It was found that the predictions of the proposed correlation are within ±5% agreement with the experimental data. In the numerical investigation, a mathematical model was developed by applying laws of conservation of mass, momentum, and energy to a straight diabatic capillary tube. These differential equations were solved numerically using a finite difference method. The developed model was validated with experimental data from previous and present research. Further, a comparison was also made between the proposed model and the models of Sinpiboon and Wongwises (2002) and Valladares (2007). © 2008 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.