speed through a fixed bed reactor containing the chelate forming ion exchanger and provided with a heatable jacket as well as the reaction temperature are preferably coordinated in such a manner with one another that the desired acrolein conversion is achieved with a single passage of the reaction mixture through the reactor after separation of the ion exchanger which usually takes place by means of sedimentation or filtration or results by itself when using a resin bed as is customary for example in water preparation the reaction mixture is freed to the extent necessary of non reacted acrolein the separation of the acrolein can be realized in a known manner especially by means of distillation preferably under reduced pressure and temperatures below 80 u00b0 c the recovered acrolein can be fed back into the process after stabilization the practically acrolein free hydroxypropionaldehyde solution obtained can be reconcentrated before hydrogenation e g via a thin layer evaporator the catalytic hydrogenation of the 3 hydroxypropionaldehyde in liquid phase is carried out in a known manner and in customary hydrogenation apparatus the catalyst can be used in suspended form per se or carrier bound or can be present in a fixed bed reactor homogenous catalysts can also be used suitable suspension catalysts are raney nickel which can be doped with various other metals as well as finely distributed platinum on a carrier material such as activated carbon among the fixed bed catalysts the substances cited in the evaluation of u s pat no 2 434 110 are used nickel catalysts have proven to be especially effective catalysts in order to achieve a high conversion rate the hydrogenation is carried out under pressure and at elevated temperature and the.