The technique of secondary ion mass spectrometry (SIMS) using an ion beam with MeV-energy has been developed to visualize the spatial distribution of biological tissues and cells with high spatial resolution. In this analytical technique, low vacuum is essential for keeping the samples wet. The effect of evaporation of volatile samples on the SIMS spectra was investigated under vacuum pressures of 50 and 500 Pa by using two higher alcohols, 1-decanol and 1-dodecanol. The results indicated that secondary ions of monomer and multimer species were generated by different phases in the sample. Moreover, the comparison of mass spectra obtained under the vacuum conditions of 50 and 500 Pa separately demonstrated that the secondary ion yields were affected by the gas flow adjusting the vacuum, whereas analysis at lower vacuum conditions allowed obtaining constant secondary ion intensity from volatile samples. The results clearly indicated the advantage of ambient analysis with the MeV-SIMS apparatus.