Time-of-flight secondary ion mass spectra were recorded from submonolayer thin films of aminopropyl end-capped poly(dimethylsiloxane) (PDMS) and from thick films (ca. 50 mu m) of poly-[(dimethylsiloxane)-urethane] (PU-DMS)-segmented copolymers. Ions detected and assigned to fragments in the low-mass range (m/z less than or equal to 300) provided structural information about the repeat units and the end groups. The high-mass spectrum of the PDMS homopolymer yielded a series of ions assigned to Ag+-cationized oligomers; this enabled determination of the molecular weight distribution in comparison with GPC measurement. In the high-mass (m/z = 800-3500) spectra of thick PU-DMS films, the peak series was assigned to a simple fragmentation process. That process would yield ions where the intact PDMS segment is present; it therefore can be used to evaluate the PDMS segment length distribution at the surface of the copolymer. It was found that the distribution of PDMS segment lengths segregated at the surface of the thick film was almost identical with that in the bulk for PU-DMS with PDMS nominal molecular weight of ca. 1000 Da. These results allow the development of an analysis of ion structure and a stepwise procedure for evaluating the segment length distributions in the near-surface region of siloxanes.