Trimethylolpropane is isolated from a reaction mixture which has been obtained by reaction of n-butyraldehyde with formaldehyde in the presence of a base and, if appropriate, hydrogenation of the resulting mixture by distilling the mixture by means of a dividing wall column or a distillation column with a precolumn or a distillation column with an after-column. The process has low capital costs and a low energy consumption and gives reduced formation of high-boiling by-products.