We report experiments highlighting the mechanistic role of mobile pentacene precursors in the formation of a network C-60-pentacene co-crystalline structure on Ag(111). This co-crystalline arrangement was first observed by low temperature scanning tunneling microscopy (STM) by Zhang et al. (Zhang, H. L.; Chen, W.: Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chest. Sac. 2008, 130, 2720-2721). We nosy show that this structure forms readily at room temperature from a two-dimensional (2-D) mixture. Pentacene, evaporated onto Ag(111) to coverages of 0.4-1.0 ML, produces a two-dimensional (2-D) gas. Subsequently deposited C-60 molecules combine with the pentacene 2-D gas to generate it network structure, consisting of chains of close-packed C-60 molecules, spaced by individual C-60 linkers and 1 nm x 2.5 nm pores containing individual pentacene molecules. Spontaneous formation of this stoichiometric (C-60)(4)-pentacene network from a range of excess pentacene surface coverage (0.4 to 1.0 ML) indicates it self-limiting assembly process. We refine the structure model for this phase and discuss the generality of this co-crystallization mechanism.