The role of a co-catalyst in the intensification of rates of liquid-liquid (L-L) phase transfer catalysed reaction is studied in allusion to the synthesis of p-chlorophenyl acetonitrile by using tetrabutylammonium bromide (TBAB) as the phase transfer (PT) catalyst and potassium iodide as the co-catalyst. Due to the co-catalytic action of potassium iodide, there is an intensification of reaction rates by orders of magnitude resulting from the in situ formation of a more active p-chlorobenzyl iodide and the selectivity is 100%. The catalyst is predominantly distributed in two forms, Q(+)1(-) and Q(+)CN(-). A mechanistic model consistent with the kinetics of the process is described. The study provides a deeper insight into the complexities of the co-catalysed phase transfer catalytic (PTC) processes. (C) 2002 Elsevier Science B.V. All rights reserved.