Allyl alcohol, particularly from biobased sources such as glycerol, is hydroformylated to products including 4-hydroxybutyraldehyde and 4-hydroxy-2-methylpropionaldehyde by forming a homogeneous reaction mixture including allyl alcohol, a rhodium-based hydroformylation catalyst and a near critical liquefiable petroleum gas or mixture of such gases, reacting the near critical liquefiable petroleum gas (or gas mixture)-expanded allyl alcohol substrate with carbon monoxide and with hydrogen in the presence of the catalyst, and recovering substantially all of the petroleum gas or gases overhead by reducing the pressure and degassing the product mixture. Dense propane is especially useful as a single inert solvent/diluent, and substantially no other solvent/diluent is needed.