A narrow-band spectroradiometer has been used to make nighttime measurements of the Phase 50 eruption of Pu'u O'o, on the East Rift Zone of Kilauea Volcano, Hawaii. On February 19, 1992, a GER spectroradiometer was used to determine the cooling rate of an active lava flow. This instrument collects 12-bit data between 0.35 to 3.0 &mgr;m at a spectral resolution of 1--5 nm. Thirteen spectra of a single area of a pahoehoe flow field were collected over a 59 minute period (21:27--22:26 HST) from which the cooling of the lava surface has been investigated. A two-component thermal mixing model [Flynn, 1992> applied to data for the flow immediately on emplacement gave a best-fit crustal temperature of 768 ¿C, a hot component at 1150 ¿C, and a hot radiating area of 3.6% of the total area. Over a 52-minute period (within the time interval between flow resurfacings) the lava flow crust cooled by 358 ¿C to 410 ¿C at a rate that was as high as 15 ¿C min-1. Our observations have significance both for satellite observations of active volcanoes and for numerical models of the cooling of lava flows during their emplacement. ¿ American Geophysical Union 1992