When opposed-jet mixers are employed to achieve rapid mixing in processes such as pharmaceutical precipitations, an unambiguous value of the mixing time is needed for comparison to characteristic times such as the induction time for crystal nucleation. We used selectivity in the fourth Bourne reaction to assess mixing quality in two types of jet mixers: Y mixers and a confined impinging jet (CIJ) mixer. We developed an equal-volume mixing model based upon Baldyga and Bourne (1999a) to interpret the reaction selectivity results in terms of a Damkohler number, defined as the ratio of an unambiguously defined physical mixing time to the characteristic reaction time. A value of unity for the Damkohler number corresponded to a reaction selectivity of 0.05, which has often been assumed in the past (Mahajan and Kirwan, 1996; Johnson and Prud'homme, 2003a). The results showed that both mixers provided rapid mixing of the order of 10 ms at jet velocities of 10m/s. Experimental mixing times were correlated with jet velocity, diameter, and viscosity using simplified turbulent mixing theories, particularly the Corrsin model, which includes both meso- and micro-mixing terms. The relevance of a combined meso- and micro-mixing mechanism in these devices was also supported by recent CFD simulations in a CIJ mixer (Liu and Fox, 2006).