The preparation of N12-(7-chloro-4-quinolinyl)-N1,N1-diethyl-1,12-diaminododecane, AQ-40, was accomplished by a five-step process in 80% overall yield from 12-aminododecanoic acid and 4,7-dichloroquinoline. AQ-40 crystallizes as a monohydrate from reagent grade chloroform/ diethyl ether mixtures in the triclinc space group P-1 with a = 8.667(2), b = 8.9425(10), c = 17.217(3) A, α = 99.34(1), β = 99.89(2), γ = 91.56(1)°, V = 1295.0 A3 and Z = 2. The l2-(N1,N1-diethylamino)dodecyl side chain is in the fully extended conformation and the water molecule forms hydrogen bonds to the two tertiary nitrogen atoms as well as with the secondary amino group. The nitrogen of the secondary amino group bound to the four-position of the quinoline moiety is virtually planar. This together with the rather short C–N distance of 1.347(3) A to the quinoline moiety suggests involvement of the lone pair on this nitrogen with the π system of the ring.