Borohydride reductions, although common, have not been studied from kinetic and mechanistic view points due to complex chemistry and more so, there is no report on kinetics and mechanism of such reductions under phase transfer catalysis. The novelties of phase transfer catalysed chemoselective reduction of citronellal to citronellol using sodium borohydride were studied in detail in liquid-liquid phase transfer catalysis using tetrabutylammonium bromide (TBAB), tetrapropylammonium bromide (TPAB), tetraethyl ammonium bromide (TEAB) and tetrabutylammonium hydrogen sulfate (TBAHS). TBAB was the best catalyst at 30 degrees C. Citronellol is very extensively used in a number of perfume compositions. The effects of different parameters such as speed of agitation, phase volume ratio, catalyst concentration, sodium borohydride concentration, citronellal concentration and temperature have been studied. The reaction was found to be 100% selective towards the formation of the desired product. The kinetics and mechanism are presented to account for the effects of various parameters on conversion and rates. It was possible to establish both rate constant and distribution constant from the same set of data. The results are novel. (c) 2005 Elsevier B.V. All rights reserved.