The reaction chemistry and kinetics of the low temperature hydroformylation of the water-soluble substrates, 4-penten-1-ol and 3-buten-1-ol, in aqueous solution utilizing the catalyst HRh(CO)(TPPTS)(3) is reported where, TPPTS is trisulphonated triphenylphosphine. The reactions were carried out at relatively low temperature and high stirring rates (1700 rpm) to maintain similar rates of mass transfer of gases into the aqueous phase. Activation parameters and reaction selectivity for the hydroformylation of 4-penten-1-ol are found to be dependent on solution ionic strength. At [Rh] = 5 x 10(-4) M, an activation energy for the hydroformylation of 4-penten-1-ol of 23 kcal mol(-1) is estimated. As sodium sulfate is added to the catalytic phase the activation energy increases. The dependence of reaction selectivity on ionic strength is unexpected. The reaction can be directed to yield a product distribution of modest linearity (75%) or an exceptionally high ratio of the branched product. observed exclusively as a cyclic 2-hydroxy-3-methyltetrahydropyran (98%), by control of solution ionic strength and temperature. A wider range of selectivities can be obtained for 4-penten-1-ol in water with HRh(CO)(TPPTS)3 than can be obtained in toluene with HRh(CO)(PPh3)(3). The hydroformylation of 3-buten-1-ol is less sensitive to reaction conditions. An activation energy of 25 kcal mol(-1) is estimated at [Rh] = 2.5 x 10(-4) M. Reaction selectivity favors in this case the formation of a six-membered over a five-membered cyclic acetal and is not influenced by temperature or solution ionic strength. It appears that the hydroxy group can direct the site of CO addition to alkenols, perhaps by chelate formation, and that solution ionic strength affects its ability to do so. (C) 2004 Elsevier B.V. All rights reserved.