Readily prepared and bench-stable rhodium complexes containing methylene bridged diphosphine ligands, viz. [Rh(C6H5F)(R2PCH2PR2')][BAr4F] (R, R' = Bu-t or Cy; Ar-F = C6H3-3,5-(CF3)(2)), are shown to be practical and very efficient precatalysts for the intermolecular hydroacylation of a wide variety of unactivated alkenes and alkynes with beta-S-substituted aldehydes. Intermediate acyl hydride complexes [Rh((Bu2PCH2PBu2)-Bu-t-Bu-t)H{kappa(2)(S,C)-SMe(C6H4CO)}(L)](+) (L = acetone, MeCN, [NCCH2BF3](-)) and the decarbonylation product [Rh((Bu2PCH2PBu2)-Bu-t-Bu-t)(CO)(SMePh)](+) have been characterized in solution and by X-ray crystallography from stoichiometric reactions employing 2-(methylthio)benzaldehdye. Analogous complexes with the phosphine 2-(diphenylphosphino)benzaldehyde are also reported. Studies indicate that through judicious choice of solvent and catalyst/substrate concentration, both decarbonylation and productive hydroacylation can be tuned to such an extent that very low catalyst loadings (0.1 mol %) and turnover frequencies of greater than 300 h(-1) can be achieved. The mechanism of catalysis has been further probed by KIE and deuterium labeling experiments. Combined with the stoichiometric studies, a mechanism is proposed in which both oxidative addition of the aldehyde to give an acyl hydride and insertion of the hydride into the alkene are reversible, with the latter occurring to give both linear and branched alkyl intermediates, although reductive elimination for the linear isomer is suggested to have a considerably lower barrier.