Experiments have been carried out on the direct initiation of detonation by means of a jet of hot product gases, produced in a constant volume explosion, which is vented into another mixture through an opening. Stoichiometric mixtures of C2H 4, C2H 2, H 2, and C3H s with oxygen and with different degrees of dilution with nitrogen were used. The initial pressure of the mixtures was 1 atm in all cases. The orifice opening was initially closed by a diaphragm set to rupture at about the peak constant volume explosion pressure of the product gases prior to venting. The results are highly reproducible and the phenomenon is not affected by the fine-scale turbulence generated by the diaphragm fragments. The results suggest that under optimal conditions of initiation there exists a minimum value of the ratio of the orifice diameter d to the cell size X of the mixture to be detonated. This minimum value d/)~ is comparable to that for detonation transmission or the critical tube diameter situation (i.e., d/X = 13). However, if the free radical concentration is high, the critical value of d/h at which initiation occurs increases. The effect of orifice geometry on the initiation of detonation is similar to that for detonation transmission. For example, the result for detonation initiation via a square orifice of identical cross-sectional area is about the same as for circular orifice, while rectangular slits are more effective in that detonation in less sensitive mixtures can be initiated. This trend corresponds identically to that observed in our earlier work for detonation transmission through similar orifices.