Based on the critical solution temperature (CST) of non-ionic phosphine ligand, a thermoregulated phase-separable catalyst formed in situ from P[p-C6H4O(CH2CH2O)(n)H](3) (PETPP, n = 10) and RhCl(3)(.)3H(2)O was applied for the first time in the hydroformylation of diisobutylene. It was found that the reaction temperature, total pressure and reaction time, as well as the P/Rh molar ratio had great influence on the reactivity of the catalyst. Under the optimum conditions, the conversion of diisobutylene and yield of aldehyde are 93.1 and 82.5%, respectively. Recycling of the PETPP/Rh complex catalyst up to three times without loss of activity has been observed. (C) 2002 Elsevier Science B.V. All rights reserved.