The heats of formation at 0 K (Delta(f)H(0)) of 29 small (containing up to 3 heavy atoms) open-shell molecules, with accurately known experimental values, have been calculated using a number of high-level theoretical procedures. The theoretical methods examined include variants of Gaussian-n (G2, G2-RAD(RMP2), G2-RAD(B3-LYP), G2-RAD(QCISD), G3, G3-RAD, G3X, G3X-RAD, G3(MP2), G3(MP2)-RAD, G3X(MP2) and G3X(MP2)-RAD), CBS (CBS-APNO, CBS-Q, CBS-RAD and CBS-QB3), and Martin extrapolation (Martin-2, Martin-3, W1, W1', W1h, W2h and W2) procedures. The open-shell systems include doublet radicals (.BeH, .CH, .CH3, .NH2, .OH, .SiH3, .PH2, .SH, .N-2(+), .NO, .ONO, .O-2(-), .CN, .CO+, .CS+, .CCH, .CHO, .OOH, .CHCH2, .CH2CH3, .CH2OH, .OCH3, .SCH3 and .COCH3) and triplet biradicals (:CH2, :NH, :SiH2, :O-2, and :S-2). The results for these systems are used to assess the performance of the various theoretical methods. The smallest mean absolute deviations (MADs) from experiment are found with the G3-RAD, G3X-RAD, G3X, W1h, W2, W1, and W2h procedures with MADs lying in the range 2.0-2.5 kJ mol(-1). The smallest values for the largest deviation (LD) from experiment are found with the G3X, G3X-RAD, W2, and W2h procedures and are +/-6.4 kJ mol(-1). A selection of the most accurate theoretical procedures (G3-RAD, G3X-RAD, G3X, W1h, W2, W1, and W2h) is used to predict the heats of formation for several radicals (.OF, .CH2CHCH2, .CH2CN, .CH2COOH, and .CH2C6H5) for which there are greater uncertainties associated with the experimental values.