There is great difficulty to obtain,molecular ions from the lubricant actually as films on magnetic recording disks. With the assistance of Ag cationization, quasi-molecular ions of (Ag + M)(+) from Fomblin Z-DOL cast on Ag substrate were successfully observed on the positive time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectra. Considering the mass interference between oligomers, detailed distributions of molecular weight (M-n) and monomer ratio (m/n) could be extracted. The average values of M-n and m/n obtained by ToF-SIMS are in good agreement with other conventional technique such as nuclear magnetic resonance spectroscopy (NMR). (C) 2002 Elsevier Science B.V. All rights reserved.