The effect of geometrical parameters, including helical pitch and wire diameter, on the fluid mixing inside a novel mesoscale helical baffled design of an oscillatory baffled reactor (OBR) was for the first time experimentally investigated at a net flow rate of 1.72 ml/min (net flow Reynolds numbers Re-n of 7.2). The results show that the influence of helical wire diameter on the fluid mixing was negligible at Strouhal numbers above 0.2. However, the degree of plug flow increased 2-fold at oscillatory Reynolds numbers beyond 300 and Strouhal number below 0.2 when the wire diameter increased. The fluid mixing was found to be a function of the ratio of oscillation amplitude to helical pitch. The results showed that plug flow behaviour can be achieved when the ratio of oscillation amplitude to helical pitch was in a range of 0.2-0.6. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.