After appropriate sample pretreatment and derivatization, uremic serum was investigated by combined high resolution gas chromatography and mass spectrometry, using both electron impact and chemical ionization methods. Electron impact and chemical ionization spectra of a number of identified (trimethylsilylated) carbohydrates and organic acids are compared. The utilization of chemical ionization mass spectrometry, with isobutane as the reagent gas, is discussed in detail. The influence of the reagent gas pressure on the total ion current and on the spectral appearance was studied. The identification of compounds, based on electron impact mass spectral data, was confirmed and often aided appreciably by using this technique. The chemical ionization spectra of trimethylsilylated alditols and aldonic acids, as well as of other organic acids showed protonated molecular ions, whereas aldoses did not. Differences with electron impact spectra are found mainly in the high mass region. The loss of one or more trimethylsilanol groups becomes the predominating fragmentation route at higher reagent gas pressure.