Sellmeier parameters of thirteen tetragonal (space group I -4) double tungstate and double molybdate laser crystals with M+T3+(X6+O4)2 composition have been calculated using the room temperature refractive indices determined from the ultraviolet band gap of the crystals to λ ≈2 μm. All considered crystals are uniaxial but only crystals with Bi in their composition show a significant birefringence(�n > 10−2). The refractive index value increases for the sequence T3+ = La, Y, Gd, Lu, and Bi independently of the M+ and X6+ cation pair. Implications for the design of laser waveguides and laser pulse dispersion are discussed.