The film former is in mass percentage the main component in most glass fibre sizes. Its influence on the fibre-matrix adhesion and the compound properties is studied in two thermoplastic matrices, i.e. polypropylene (PP) and rubber-modified styrene-co-maleic anhydride. The composition of the size is varied by changing the chemical nature and molecular mass of the film former and by leaving out the silane. The size＇s morphology on the glass fibres is studied with scanning electron microscopy and X-ray photoelectron spectroscopy. The effect on fibre-matrix adhesion is inferred from 0 degrees bending and interlaminar shear strength measurements of unidirectional, continuous fibre reinforced composites. The possible effect of the film former on matrix crystallinity in PP is studied with differential scanning calorimetry and polarized-light microscopy. The same sizes are also tested in injection moulded, short fibre reinforced compounds by measuring their mechanical properties. In addition, the diffusion of the size into the matrix is studied by confocal scanning laser microscopy. The main conclusion of this study is that the film former plays a crucial role in the level of fibre-matrix adhesion because it does not diffuse away from the fibre-matrix interface into the matrix. Its effect may be larger than that of the silane. In addition, most mechanical properties of short fibre reinforced compounds improve with increasing fibre-matrix adhesion.