Thermal damage of silicon photodiodes exposed to intense optical radiation is investigated. Damage thresholds of Si photodiodes irradiated by 1.06μm laser pulses are reported for values of irradiation time,τ, ranging from 10−8 to 1s. Threshold laser irradiation produces visible microscopic damage and a permanent degradation in photoresponse. The loss of responsivity is associated with degradation of the detector diode characteristics due to laser-induced heating. The time and wavelength dependence agree with the predictions of a thermal model which treats a semi-infinite material irradiated by a Gaussian laser beam. The energy density thresholds are independent ofτ for short irradiation times and asymptotically approach a limiting behaviour which increases asτ for long times. They are given by the empirical relationE0=65[1+217τ/tan−1(258τ)1/2] J cm−2 for 1.06μm radiation. The thresholds at short irradiation times of detectors damaged by 1.06μm radiation are about 25 times larger than those of detectors exposed to 0.6943μm radiation. The greater susceptibility at 0.6943μm is attributed to a larger optical absorption coefficient.