The mixing behavior at different Reynolds numbers is analyzed for different T-shaped micro-channels with a rectangular cross-section. The focus is on analyzing the effect of aspect ratio on the critical Reynolds number at which engulfment occurs. Soleymani et al. (2008) have recently proposed an empirical equation based on extensive numerical simulations to determine this critical Reynolds number. This is valid only when the depth of the channels (C) is lower than the widths of the mixing (A) and inlet (B) channels i.e. when A/. C and B/. C are both greater than unity. We find that when they are both less than unity there is a delay or even absence of engulfment even when Re is increased. The predictions of these simulations are validated experimentally for two geometries. © 2010 Elsevier Ltd.