The importance of hydroformylation reactions for fundamental organic and industrial synthesis has prompted a detailed analysis of the factors governing the regioselectivity of these processes. A simple 2(k) factorial design has shown that a very significant increase in the regioselectivity of vinyl-aromatics (typically from 70% to more than 98%) can be obtained by the exclusive manipulation of two variables, pressure and temperature, within the usual working values for such variables. It is also shown that optimised values can be obtained from within a range of possible values. The role of the ligand in the overall response of the system is also discussed and analysed. (c) 2006 Elsevier B.V. All rights reserved.