Bicyclic guanidines are prepared by reacting dialkylenetriamines with dialkylcarbonates in the presence of a silane of the formula
Si(OR
x
)
o
R
Y
(4−o)
(IV)
and/or their partial hydrolysates, with the proviso that contain minimally one unit IV which is a monovalent optionally substituted hydrocarbon radial with 3 to 10 carbon atoms.