The effect of acetone vapours on the phase transformation x --> alpha was studied by means of electron microscopy, X-ray diffractometry, electron spin resonance (ESR) and optical spectroscopy on thin films and powders of lithium phthalocyanine (PcLi). The exposure of amorphous films of PcLi, deposited at a substrate temperature of 15 degrees C onto glass slides, to solvent vapours of acetone at room temperature, induces important changes in the morphology and crystalline structure of the films. Exposure to acetone vapours under low pressure (200 mbar) leads to an x-like structure with low crystallinity, uniform coverage and no preferential ordering. Longer exposure in a saturated acetone vapour atmosphere results in an important phase transformation to the ct form which leads to crystallites up to several micrometres large lying in the plane of the substrate and forming a discontinuous coverage. In the case of x powders treated in liquid acetone, a phase transformation to the alpha form of PcLi is observed. This x --> alpha phase change induced by the acetone vapour treatment is interpreted in terms of crystal size effects and compared with the phase transformation observed in thin films deposited at various substrate temperatures in the range 25-225 degrees C.