The microstructure of (In, Mn)As thin films grown using metalorganic vapor phase epitaxy (MOVPE) was investigated to determine the origin of room temperature ferromagnetism in these films. Transmission electron microscopy based techniques were used to investigate phase purity and compositional homogeneity. Microanalysis of an In1-xMnxAs film, with x = 0.01 and a Curie temperature of 330 K exhibited a homogeneous distribution of Mn. High Mn concentration films with x = 0.20 exhibited MnAs precipitates within the (In,Mn)As matrix. The analysis indicates that room temperature ferromagnetic, single-phase (In,Mn)As can be formed by MOVPE. The origin of ferromagnetism is attributed to (In,Mn)As solid solution rather than distinct secondary Mn-rich magnetic phase(s). (C) 2003 Elsevier B.V. All rights reserved.