Detection of volatile organic compounds (VOCs) in aqueous solution at low parts-per-trillion (ppt) levels is accomplished using a very simple and efficient on-line preconcentration cryotrap membrane introduction mass spectrometry (CT-MIMS) system. The conventional MIMS probe is modified so that the membrane interface is placed about 15 cm away from the ion source. A U-shaped trap tube is then inserted between the membrane interface and the ion source. Cryotrapping is performed with liquid nitrogen for 15 min, followed by fast heating at ∼15°C s^-1, which thermally releases the condensed VOCs almost at once into the ion source region of a quadrupole mass spectrometer. By applying electron ionization and a selective ion monitoring scan mode, a very sharp and intense peak is obtained. The performance of the CT-MIMS system was compared with that of conventional MIMS, and after reaching the best conditions for the trapping and heating cycles, an improvement factor in signal intensity of about 100 was observed for a series of VOCs. The extraordinary sensitivity of CT-MIMS system allows VOCs to be detected at very low concentrations, detection limits being typically on the order of 10-20 ppt. The results also show excellent linearity and reproducibility for the system.