This paper investigates open-loop control of the mixing between an inner heated stream and an outer unhealed stream in coaxial combustor geometry. The inner and outer streams mimic the gaseous fuel and primary air stream in a gas turbine combustor. Of specific interest in this study is the behavior of the coherent structures, and the mixing in these coherent structures under the effect of controlled perturbation of the primary air stream. Two perturbation or forcing frequencies are studied: the preferred mode or fundamental frequency of the unforced flow which is measured to be 500 Hz, and the subharmonic frequency of 250 Hz. At each forcing frequency, three different forcing levels were examined. At the high-forcing level, the preferred mode forcing produces an increase in the spreading rate of both the inner and the outer jets. This is associated with increased entrainment into the coherent structures and increased mixing of the heated inner and unheated outer jet. With subharmonic forcing, a similar behavior is observed due primarily to the emergence of the fundamental as a dominant mode. At low-level forcing, the velocity profiles are only marginally affected, but the temperature profiles indicate a decrease in mixing. It is speculated that this is associated with increased coherence but without an associated increase in entrainment into the coherent structures.