Regioselectivity control was studied in palladium catalyzed hydroxycarbonylation of styrene in neat water with water-sol. phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also N-bis(N',N'-diethyl-2-aminoethyl)-4-aminomethylphenyl-diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temp. In the N3P case, only a minor variation in the n/i ratio as a function of temp. is obsd. In situ normal- and high-pressure NMR expts. were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium η3-benzylic complex and both the branched and linear palladium acyl complexes were identified in the HP NMR expts. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium-hydride bond gives an η3-benzyl compd. A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temps., the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low n/i therefore requires high pressures and reasonably low temps. The N3P ligand always favors the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.