Several polynuclear iron(III)-o-phthalates precipitated from acid and alkaline solutions were characterized by chemical and thermogravimetric analysis, and infrared, Mossbauer and magnetic measurements. Compounds prepared in acid solutions contain the trimer unit Fe3O and, although different in chemical composition and in thermal decomposition behaviour, show very similar IR, Mossbauer and magnetic characteristics. The variable temperature magnetic susceptibility measurements of these compounds were fitted to a theoretical expression derived from a spin Hamiltonian, taking into account two different exchange pathways along inequivalent sides of an isosceles triangle, yielding J(12)/cm(-1) = -35.4 and J(13)/cm(-1) = -21.1. The Mossbauer spectra of this group of samples could be satisfactorily fitted by considering the superposition of two quadrupole split doublets with different relative areas. Samples prepared in highly alkaline solution show a characteristic band in the infrared spectra at 890 cm(-1), assigned to the asymmetric Fe-O-Fe stretch. Their Mossbauer spectra taken in the temperature region 77-300 K reveal superparamagnetic behaviour of small particles or clusters. Magnetic susceptibility measurements of these polynuclear iron(III) species show T-max/K = 10 and 25, respectively. Zero field-cooled, field-cooled and torque measurements were used to prove their spin glass behaviour in the low temperature region.