Critical tube diameters d c for the successful transformation of a planar to a spherical detonation have been measured in nine gaseous fuels (CH 4, C 2H 2, C 2H 4, C 2H 6, C 3H 6, C 3H 8, C 4H 10, MAPP and H 2) in stoichiometric fuel-oxygen mixtures diluted with nitrogen at atmospheric initial pressure. In agreement with the previous work of Matsui and Lee. acetylene is found to have the smallest critical tube diameter of 12 cm at stoichiometric composition with air. However, in contrast with the earlier estimate of Matsui and Lee, hydrogen is now found to be the second most sensitive fuel with a critical tube diameter of 20 cm. Both ethylene and MAPP, with critical diameters of about 38 and 53 cm, respectively, have nearly the same sensitivity. The heavier hydrocarbons (i.e., C 4H 10, C 2H 6, C 3H 6, and C 3H 8) are found to belong to the same sensitivity class with critical diameters of 64, 67, 70, and 70 cm, respectively. Direct measurements of detonation cell sizes λ in both atmospheric fuel-oxygen-nitrogen mixtures and subatmospheric fuel-oxygen mixtures demonstrate that the observation of Mitrofanov and Soloukhin in low-pressure ( p 0 ⋍ 80 Torr) C 2H 2O 2 mixtures that d c3- 13 λ is valid for other fuels as well and can be used as an empirical law to estimate critical tube diameter from cell size data. Based on the preseent results for the critical tube diameter, initiation energies have been estimated using the work-done formula of Lee and Matsui to evaluate the detonation hazard numbers D H as proposed previously by Matsui and Lee. For stoichiometric fuel-air mixtures at 1 atm initially, acetylene ranks first ( D H = 8.4 × 10 5), followed by hydrogen ( D H = 3.4 × 10 6) and ethylene oxide ( D H = 1.1 × 10 7) and MAPP ( D H = 7.2 × 10 7) have nearly the same sensitivity. The other alkanes ( D H = 1.5 × 10 8 for ethane, D H = 1.7 × 10 8 for n-butane, D H = 1.73 × 10 8 for propane) and the alkenepropylene ( D H = 1.6 × 10 8) belong to the same sensitivity class.