Poly(dimethylsiloxane) (PDMS) cross-linked films with different concentrations of micrometer-sized garnet particles were prepared through hydrosilylation reaction between a di-vinyl-terminated PDMS and a four-functional silane cross-linker in the presence of a Platinum catalyst. The garnet particles, consisting of micrometer-sized Ce doped Y3Al5O12 particles commonly used on light converters (for solid state light devices, e.g. Light emitting diodes LEDs), were homogeneously and individually dispersed through the bulk of the cross-linked films for a concentration up to 20% in volume. The effect of the garnet on the cross-linking of the composite films was investigated by in-situ ATR-FTIR Spectroscopy. When formulations with an excess of cross-linker were used the overall consumption of Si-H groups of the cross-linker is slowdown in the presence of the garnet. The same effect was observed for an increasing concentration of the garnet. This effect is attributed to a strong interaction of the catalyst with the garnet during the cross-linking process. The possible origins of this interaction are discussed. NMR Imaging was used to investigate the cross-linked networks formed in the composite films. All the networks are homogeneous through the depth of the cross-linked composite films (approx. 5 mu m resolution). The films prepared in the presence of the garnet particles showed similar relaxation behavior, regardless of the cross-linker concentration used in the initial formulation, contrary to previous results with PDMS cross-linked films. This behaviour is explained based on the presence of different type of chemical cross-links, resulting from the occurrence of hydrosilylation and secondary reactions, which are influenced by the interaction between the garnet particles and the catalyst. (C) 2009 Elsevier Ltd. All rights reserved.