Dialkyltriselenadiborolanes I [R = Et (II), Pr (III)] react at room temp. with excess N-bases [pyridine (py), 3,5-dimethylpyridine (Me2py), 3-chloropyridine (clpy), quinuclidine (qn)] to form 2:1 adducts [py2.II, py2.III, (clpy)2.II, (me2py)2.II, qn2.II, qn2.III]. A variable temp. 1H and 11B NMR study on qn2.II suggests the existence in soln. of a mixt. of syn/anti isomers. At -80 Deg, adduct (syn)-qn2.II is in equil. with qn.II and qn. X-ray anal. of qn2.II shows two N-bases in an anti configuration on the B2Se3 heterocycle. At room temp. in soln. with equimolar quantities of N-base, only the 1:1 adducts are formed in which only one boron atom of I is tetracoordinated. Formation of 1:1 adducts, in which the N-base fluctuates between the two boron atoms of I may be obsd. when an equimolar soln. of I and N-base are heated to 120 Deg. Hexamethylenetetramine forms solid 1:1 and 2:1 adducts with II.