The two-group two-fluid model with interfacial area transport equation has been developed to improve the prediction of void fraction and heat transfer characteristics in two-phase systems. In the one-dimensional formulation, a closure relation is required for the group-1 and group-2 area-average local relative velocity. Furthermore, in the case of the modified two-fluid model with the gas-mixture momentum equation, the group-1 and group-2 void weighted gas velocities must be calculated with additional closure relations. The drift-flux general expression is extended to two bubble groups in order to describe the group-1 and group-2 void weighted gas velocities and area-averaged local relative velocities. Correlations for group-1 and group-2 distribution parameters and drift velocities are proposed and evaluated with a two-group boiling dataset taken in an internally heated annulus. The proposed distribution parameters show an average agreement within +/- 5%. The overall estimation of group-1 and group-2 void weighted gas velocities calculated with the newly proposed two-group drift-flux general expression shows an average agreement within +/- 16% of the measured value. The equations obtained for area-averaged relative velocity of group-1 and group-2 bubbles were simplified by neglecting covariance in void fraction. This assumption was compared with the experimental database and resulted in an average error within +/- 13%. (C) 2012 Elsevier Ltd. All rights reserved.