A process for preparing triethylenediamine by passing an amine compound over a catalyst at elevated temperature to afford a reaction product containing triethylenediamine and piperazine, the amine compound having, in the molecule, a moiety represented by the following general formula ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms and X is oxygen or nitrogen characterized by adding an ethylating compound containing at least one nitrogen and/or oxygen atom to the reaction product and contacting the reaction product, under conditions sufficient to produce triethylenediamine from the ethylating compound and piperazine, with a condensation/cyclization shape-selective zeolite catalyst demonstrating a triethylenediamine/piperazine weight ratio uptake value of at least 6:1.