The highly unusual structural and electronic properties of the alpha-phase of (Si(1-x)C(x))(3)N(4) are determined by density functional theory (DFT) calculations using the Generalized Gradient Approximation (GGA). The electronic properties of alpha-(Si(1-x)C(x))(3)N(4) are found to be very close to those of alpha-C(3)N(4). The bandgap of alpha-(Si(1-x)C(x))(3)N(4) significantly decreases as C atoms are substituted by Si atoms (in 2 most cases, smaller than that of either alpha-Si(3)N(4) or alpha-C(3)N(4)) and attains a minimum when the ratio of C to Si is close to 2. On the other hand, the bulk modulus of alpha-(Si(1-x)C(x))(3)N(4) is found to be closer to that of alpha-Si(3)N(4) than of alpha-C(3)N(4). Plasma-assisted synthesis experiments of CN(x) and SiCN films are performed to verify the accuracy of the DFT calculations. TEM measurements confirm the calculated lattice constants, and FT-IR/XPS analysis confirms the formation and lengths of C-N and Si-N bonds. The results of DFT calculations are also in a remarkable agreement with the experiments of other authors.