IR diode laser spectroscopy was used to detect the products of HCNO (fulminic acid) photolysis at 248 nm. Five product channels are energetically possible at this photolysis wavelength: O + HCN, H + (NCO), CN + OH, CO + NH, and HNCO. In some experiments, isotopically labeled (18)O2, (15)N(18)O and C2D6 reagents were included into the photolysis mixture in order to suppress and/or isotopically label possible secondary reactions. HCN, OC(18)O, C(18)O, NCO, DCN, and NH molecules were detected upon laser photolysis of HCNO/reagents/buffer gas mixtures. Analysis of the yields of product molecules leads to the following photolysis quantum yields: ϕ1a (O + HCN) = 0.39 ± 0.07, ϕ1b (H + (NCO)) = 0.21 ± 0.04, ϕ1c (CN + OH) = 0.16 ± 0.04, ϕ1d (CN + NH(a(1)Δ)) = 0.19 ± 0.03, and ϕ1e (HNCO) = 0.05 ± 0.02, respectively. The uncertainties include both random errors (1σ) and consideration of major sources of systematic error. In conjunction with the photolysis experiment, the H + HCNO reaction was investigated. Experimental data demonstrate that this reaction is very slow and does not contribute significantly to the secondary chemistry.