We have carried out a percolation analysis of the adsorption of ethane and nitrogen in SBA-2, a structured mesoporous silica consisting of a hexagonal close-packed (hcp) array of spherical cavities connected by cylindrical channels. Our analysis explains the different uptakes of nitrogen and ethane in terms of the greater accessibility of the network to the smaller nitrogen molecule. The analysis also allows us to quantify the connectivity of the SBA-2 pore network. The effective coordination number of the cavities, defined as the average number of channels per cavity that are large enough to allow nitrogen to pass, is 4.9, much less than the theoretical maximum value of 12. Taking into account only the smaller set of channels large enough to admit ethane, the effective coordination number is 1.8, just above the percolation threshold of the network.