A new buoyancy-modified turbulence model is developed on the basis of the four-equation model, k-epsilon-theta(2)-epsilon(theta), of Hanjalic [K. Hanjalic, S. Kenjeres and F. Durst, Natural convection in partitioned two-dimensional enclosures at higher Rayleigh numbers, Int. J. Heat Mass Transfer 39(7) (1996) 1407-1427], The strong anisotropy of Reynolds stresses due to buoyancy effects in the vertical boundary layers is considered by inclusion of the newly devised ＇return-to-isotropy＇ concept in the pressure-strain correlation. The wall-reflection functions is also duly modified. The new model has been tested in buoyancy-driven cavity flows through comparison with published experimental data and the predictions from three other turbulence models [N. Z. Ince and B. E. Launder, On the computation of buoyancy-driven turbulent flows in rectangular enclosures, Int. J. Heat and Fluid Flow 10(2) (1998) 110-117; K. Hanjalic, S. Kenjeres and F. Durst, Natural convection in partitioned two-dimensional enclosures at higher Rayleigh numbers, Int. J. Heat Mass Transfer 39(7) (1996) 1407-1427]. It has demonstrated significant improvements in capturing the non-isotropy of Reynolds stresses and turbulent heat flux in vertical boundary layers.