Compounds which are useful as sensitizers of photoconductors, having the general formula: ##STR1## in which: like symbols represent identical substituents,at least 4 of R.sup.1 to R.sup.9 represent hydrogen atoms and the remaining R.sup.1 to R.sup.9 pairs of groups independently represent:(i) hydrogen, an alkyl group having from 1 to 18 carbon atoms, a substituted alkyl group, or an aryl or substituted aryl group,(ii) a halogen atom, a cyano group, a carboxy or alkoxycarbonyl group, a hydroxy group, an alkoxy group, a nitro group or a sulfonic acid group, providing that not more than two such groups (ii) are present in each ring,(iii) an acylamino group, a secondary acylamino group or a tertiary amino group in which the pendant substituent groups on the nitrogen atom are alkyl, aryl or aralkyl, providing not more than two such groups (iii) may be present in each benzocarbazole ring, and(iv) any two groups R.sup.1 to R.sup.9 on adjacent carbon atoms in a ring may represent the necessary atoms to complete a fused alicyclic, aromatic or heterocyclic ring,R.sup.10 represents an alkyl or substituted alkyl group, an aralkyl group, or an aryl or substituted aryl group,Ar represents a heteroaromatic nucleus containing 5 or 6 members in the ring at least one of which is O, N or S, or an aromatic ring of the formula: ##STR2## in which: R.sup.11 to R.sup.15 independently represent a hydrogen or halogen atom, an alkyl, substituted alkyl, an aryl or substituted aryl, hydroxy, alkoxy or aryloxy group,one of R.sup.11 to R.sup.15 may additionally represent an aldehyde or acetyl group, or a dialkylamino group,up to two of R.sup.11 to R.sup.15 may additionally represent a cyano, carboxy, alkoxycarbonyl, sulfonic acid or nitro group, andany two adjacent R.sup.11 to R.sup.15 groups may represent the necessary atoms to complete a fused alicyclic, aromatic or heterocyclic ring, andX represents an anion.