The compositional evolution of the network structure and connectivity in binary SnO-P2O5 glasses with 35 <= mol % SnO <= 55 is studied using two-dimensional Sn-119 and P-31 NMR spectroscopy. The phosphate Q(n) species concentrations, as obtained from the analyses of the P-31 NMR spectral line shapes suggest that the network modification can be described in terms of a binary Q-speciation model. On the other hand, the Sn-119 NMR spectra suggest that the Sn-O coordination environment in these glasses is composition dependent. Sn is present in both 3-fold (trigonal pyramid) and 4-fold (trigonal bipyramid) coordinations with oxygen in these glasses. The relative fraction of the trigonal pyramidal environment increases with increasing SnO content. The unusually low glass transition temperature T-g of these glasses (241-266 degrees C) is argued to be related to the low coordination number of Sn, leading to a relatively sparsely connected structural network.