A simple and rapid method for the determination of paraffins (PA), cycloparaffins (CY), and aromatic hydrocarbons (AR) was developed by chemical ionization mass spectrometry, and applied to type analysis of condensate oils which contained little oleffins. After 0.200 ml of p-xylene-d₁₀ was added to 1.80 ml of condensate oil as an internal standard, 0.05 μl of the sample was introduced into a GC/MS equipped with a capillary column (0.25 mm i. d. × 15 m) at the split ratio of 10 : 1. The reconstructed mass spectra were obtained by monitoring the characteristic ions, such as (M±H)⁺, C_nH_2n+1⁺ etc., with a methane reagent gas. The concentrations of PA, CY, and AR were calculated from simultaneous equations equal to numbers of components to be determined, which set up by assuming additivity of the peak intensities. Analytical results were in satisfactory agreement with those obtained by the modified method of the fluorescent indicator adsorption (FIA) method for condensate oils. The measurement time by the present method was ca. 5 min in contrast with ca. 6 h of the modified FIA method.