This paper reports electron paramagnetic resonance data of substitutional Gd3+ impurities in the cubic and tetragonal phases of barium and strontium titanates. The pertinent spin Hamiltonian parameters are determined as a function of temperature in these two mentioned phases of both crystals. The temperature for the cubic-to-tetragonal phase transition in SrTiO3 is determined from these data to be 110+/-2.5°K, a result that has been checked by the observation of the temperature dependence of the birefringence pattern of pure (undoped) crystals. This experiment is also briefly described. In the latter crystal we find a smooth transition, contrary to the corresponding case in BaTiO3. The Gd3+ resonance results are compared with previous data on electron paramagnetic resonance of iron-group impurities, and some comments and conjectures are offered as to possible correlation of these results with the temperature dependence of certain properties of the host crystals in the neighborhood of the phase transition.