Electron spin-lattice relaxation (SLR) times of P1 and P2 centers have been measured at 300 K at X-band and W-band for one synthetic (type-Ib) and three natural (type-Ia) diamonds using electron paramagnetic resonance spectroscopy. Bi-exponential SLR, with a fast and a slow component, was observed for the overlapping lines of samples with two types of impurity centers. Comparison of X- and W-band results for P2 centers suggests that these centers relax via a field-independent second-order Raman process with T-1e approximate to 2.2 ms at room temperature.