Experimental structure functions for (Na(2)O)(0.35) [(P(2)O(5))(1 - x)(B(2)O(3))(x)](0.65) glasses, where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, have been measured by high-energy x-ray diffraction up to wavevectors of 28 Å( - 1) to obtain atomic pair distribution functions with high real space resolution. The experimental diffraction data have been used to guide constrained reverse Monte Carlo simulations of the three-dimensional structure of the glasses. The resulting models show that the glasses exhibit a very complex atomic-scale structure that evolves from an assembly of chains of corner shared P(O)(4) tetrahedra for x = 0 to a network of B(O)(4) tetrahedra and planar B(O)(3) units for x = 1. In the glasses of intermediate composition (i.e. 0 < x < 1), P, B and oxygen atoms sit on the vertices of P(O)(4), B(O)(4) and B(O)(3) units mixed in various proportions. Sodium atoms are found to fill up the cavities in between the P/B-oxygen units in a more or less random manner. The new data can provide a firm structural basis for an explanation of the mixed glass former effect where a nonlinear behavior of Na ion conductivity is observed in the (Na(2)O)(0.35) [(P(2)O(5))(1 - x)(B(2)O(3))(x)](0.65) glass system.