The invention provides novel persistent free radicals useful in Overhauser MRI being triarylmethyl radicals having as at least one aryl group a group of formula ##STR1## (wherein each X which may be the same or different is an oxygen or sulphur atom or a group CO or S(O).sub.n, (where n is 1 to 3) with the proviso that at least one group X is a sulphur atom or a S(O).sub.n group; R.sup.1 is a hydrogen atom or group of formula --M, --XM, --X--Ar.sup.2, or --Ar.sup.2 where M is a water solubilizing group, and Ar.sup.2 represents a 5-10 membered aromatic ring optionally substituted by a water solubilizing group M; and each of the groups R.sup.7, which may be the same or different, is a hydrogen atom, or a hydrocarbon group or a water solubilizing group M, or two groups R.sup.7 together with the atom to which they are bound represent a carbonyl group or a 5 to 8 membered cycloalkylidene, mono- or di-oxacycloalkylidene, mono- or di-azacycloalkylidene or mono- or di-thiacycloalkylidene group optionally with the ring attachment carbon replaced by a silicon atom, and R.sup.7 where it is other than hydrogen, is optionally substituted by a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group or a water solubilizing group M)). The novel radicals have good stability, long relaxation times and good relaxivity.