The absolute rate coefficient of the gas-phase reaction HCCO+O-2 was determined over the temperature range 296-839 K and at a pressure 7 +/- 1 Torr helium. The experiments were performed in a slow-flow kinetic apparatus employing pulsed photolysis of CH2CO at 193 nm asa source of HCCO radicals. Reaction time profiles of [HCCO] were constructed using a newly developed, sensitive spectroscopic technique in the visible spectral region to detect this radical: laser-induced fluorescence of nascent CH(X(2)Pi) photofragments following HCCO photodissociation at 266 nm. Photodissociation of HCCO at this wavelength was found to produce rotationally excited CH(X) populated to N"greater than or equal to 26. The rate coefficient for the title reaction was found to be described by k(T)((HCCO+O2)) = (2.6 +/- 0.3) x 10(-12) exp[-(325 +/- 80)K/T] cm(3) s(-1) molecule(-1) (2 sigma errors). The absorption cross section of HCCO at 266 nm, sigma (HCCO(266) (nm)), was also determined relative to that of CH2CO at 193 nm as sigma (HCCO(266 nm)) = 0.07(-0.05)(+0.20)sigma (CH2CO(193 nm)).