We present time-resolved pump–probe experiments in a transmission geometry using off-resonant excitation on very high-quality silicon inverse opals. We show that the nonlinear optical response can drastically be modified by tempering of the sample. The as-grown samples are dominated by an absorptive response with recovery times as short as one picosecond. For the tempered samples, both the relaxation and the scattering times increase, leading to a prominent dispersive response. The data reveal a transient blue-shift of the fundamental stop band on the order of 150 nm with transmission changes as large as a factor of five. Based on simple calculations using the Drude model we estimate corresponding refractive index changes as large as Δn = –0.5 + i0.07.