We have studied the phase and micellization behavior of a series of model surfactant systems using Monte Carlo simulations on cubic lattices of coordination number z = 26. The phase behavior and thermodynamic properties were studied through the use of histogram reweighting methods, and the nanostructure formation was studied through examination of the behavior of the osmotic pressure as a function of composition and through analysis of configurations. Our results show that only phase separation occurs for the surfactants with short head groups. Nanostructure formation then occurs for long head groups, or for short heads, where these moieties repel each other strongly. The transition from phase separation to nanostructure formation is thus quasi-continuous and can be readily tuned by varying surfactant architecture and interactions. The results obtained from these simulations, which can be qualitatively understood through theories valid for block copolymers in selective solvents, provide the first benchmark studies against which analytical theories can be compared.