Buoyant flow of an optically thick fluid representative of molten glass or an oxide crystal melt is solved via a finite element method using four different radiation models-the Rosseland diffusion approximation with and without radiation slip, the P-1 approximation, and a rigorous numerical treatment. The results indicate that both diffusion approximations fail to accurately predict thermal and flow fields in : this problem due to their inability to represent thermal boundary layers. However, the P-1 approximation matches well with the rigorous numerical solution. This accuracy, coupled with implementation ease, favors the P-1 approximation for solution of problems of this type.