In biphasic flow reactor, interphase mass transfer and associated chemical reaction is often enhanced by tuning the phase distribution to promote segmented flow. Since miniaturisation favours segmented flow, several studies are reported in reduced dimensions mostly in microchannels. Biphasic flow in millichannels is relatively less explored. Further, almost nothing is known about the influence of reaction on the flow physics. In the present study, extensive experiments are performed for reacting as well as non-reacting flow systems in a millichannel. Experiments reveal a higher conversion for vertical upflow primarily due to an enhanced range of segmented flow. Conversion is found to depend on flow morphology with segmented flow giving the maximum conversion followed by other flow patterns. The overall reaction rate constant is influenced by mixture velocity as larger values are obtained at higher velocities irrespective of the prevailing flow pattern. We observe reaction to induce phase inversion as well as transition between flow patterns along the conduit length. The phase properties are also observed to alter with reaction progress. The interplay of flow dynamics and reaction discussed in the paper is expected to aid design of milliscale flow reactors.