On magnetic recording disks, a surface lubricant film has been widely used in order to reduce friction and wear. It is very important to analyze the lubricant film on the disk directly. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a suitable technique for direct analysis of lubricant films because of its high surface sensitivity for the information about organic chemical structure. In this study, we investigated the effect of thickness and molecular weight of Z-Dol lubricants on the intensity of the Z-Dol end-group ion. ToF-SIMS results revealed that the calculated intensities of Z-Dol end-group ions from free lubricant films are proportional to free lubricant thickness, and decrease with the increase in molecular weight. By investigating the calculated intensities of Z-Dol end-group ions from free lubricant films, we concluded that it is possible to compare molecular weights of lubricant films on the disks. (C) 2004 Elsevier B.V. All rights reserved.