The rhodium-catalyzed hydroformylation of 2-vinyl-5-methyl-1,3-dioxane (VMD), the acetal derived from acrolein and 2-methyl-1,3-propanediol, has been investigated. Under homogeneous conditions, phosphane-based catalysts afforded perfect chemoselectivity for the aldehydes. Catalysts based on diphosphine ligands with a narrow bite angle of approximately 85°, such as 1,2-bis(phenylphosphino)ethane, were found to favor the branched aldehyde 2, with selectivities of up to 78 %, whereas diphosphine ligands with a large bite angle of approximately 110°, such as xantphos, produce the linear aldehyde 1 with up to 91 % selectivity. Control of the 1/2 regioselectivity by means of the PRhP bite angle was ascertained with a variety of ethylene-, hydrazino- and CNC-bridged diphosphines, irrespective of electronic factors induced by these ligands. Under aqueous biphasic conditions, chemoselective hydroformylation of VMD proved feasible only with disulfonated xantphos; in this case, remarkable activities (turnover frequencies of up to 1075 molaldehyde molRh−1 h−1 at 120 °C) were observed, with up to 82 % yield for the linear aldehyde. On the other hand, the catalyst based on tetrasulfonated dppe systematically induced the formation of very large amounts of the hydrogenated product.