Abstract: The present invention relates to tribenzylcyclen compounds of formula I ##STR1## (where R is hydrogen, or a C.sub.1-12 alkyl group optionally substituted by hydroxy, alkoxy or aryl groups or R is an amphiphilic aralkyl group comprising a N, S, O or P interrupted C.sub.2-25 alkylene chain, e.g. a polyalkylene oxide chain or R provides a bridge to a second tribenzylcyclen group, but with the proviso that R is other than benzyl; X is CHR.sub.1, or where R is hydrogen two X groups may each represent CO groups; and R.sub.1 is hydrogen, a C.sub.1-6 alkyl group optionally substituted by hydroxy, alkoxy or carboxy groups or an aralkyl group having 1 to 6 carbons in the alkyl moiety and optionally substituted in the aryl moiety by alkyl, alkoxy, hydroxy or isothiocyanate groups). These compounds are useful in the preparation of DO3A, N-substituted-1,4,7,10-tetraazacyclododecane-N',N",N"'-triacetic acids, and the phosphonic acid analogs.