A process to produce 3-methylmercaptopropionaldehyde is provided. The process comprises: A) oxidizing a mixture of propylene and inert diluent gases with air over a heterogeneous catalyst to obtain a gaseous oxidation reaction mixture comprising acrolein and by-products; B) quenching the gaseous oxidation reaction mixture to obtain a gas stream comprising acrolein and a quench liquid comprising residual acrolein and the by-products; C) stripping the acrolein from the by-products of the quench liquid and returning the stripped acrolein to the quench; D) absorbing a first portion of the gas stream comprising acrolein from the quench B) in water to obtain an aqueous acrolein solution and an uncondensable gaseous stream comprising inert gases; D1) recycling at least a portion of the uncondensable gaseous stream to the oxidation A) to supply the inert diluent gases; E) distilling the acrolein from the aqueous acrolein solution to obtain an acrolein-free aqueous bottom product; E1) condensing the distilled acrolein; F) reacting the distilled acrolein E1) and the gas stream comprising acrolein B) with methyl mercaptan in a mixture comprising at least one of 3-methylmercaptopropionaldehyde and a hemithioacetal of methyl mercaptan and 3-methylmercaptopropionaldehyde to obtain the 3-methylmercaptopropionaldehyde.