The described interface for membrane introduction mass spectrometry (MIMS) makes it possible to directly determine concentrations of organic substances in water solution without any preconcentration technique. Some nonpolar, semipolar and semivolatile substances were chosen as examples for demonstration of the MIMS function. Various kinds of membranes were tested. The detection limit, calibration curves and mutual interference of substances were determined for benzene, naphthalene and propan-1-ol. Ion current intensities were measured in the SIM mode. Hydrolysis of ethyl acetate with NaOH was chosen as an example of in-situ kinetic study using MIMS.