We have investigated the phase transition between two distinct mesoporous silicas: SBA-15, comprising a hexagonally packed arrangement of cylindrical pores (6-12 nm in diameter), and mesocellular silica foams (MCF), consisting of spherical voids (22-42 nm in diameter) interconnected by `windows' of approximately 10 nm. Both SBA-15 and MCF are formed using an amphiphilic triblock copolymer (Pluronic P123) as a template. The synthesis conditions for the two materials are identical, except substantial trimethylbenzene is added to form MCF. We find that the phase transition occurs at an oil-polymer mass ratio of 0.2-0.3. Although the pore structures and pore sizes change dramatically, the mean surface curvature of the system remains essentially the same throughout the transition.