Here we present proof-of-principle investigations on a novel inlet system for proton-transfer-reaction mass spectrometry (PTR-MS) that allows for the analysis of trace compounds dissolved in water. The PTR-MS technique offers many advantages, such as real-time analysis, online quantification, no need for sample preparation, very low detection limits, etc.; however it requires gas phase samples and therefore liquid samples cannot be investigated directly. Attempts to measure trace compounds in water that have been made so far are mainly headspace analysis above the water surface and membrane inlet setups, which both are well suitable for certain applications, but also suffer from significant disadvantages. The direct aqueous injection (DAI) technique which we will discuss here turns out to be an ideal solution for the analysis of liquid samples with PTR-MS. We show that we can detect trace compounds in water over several orders of magnitude down to a concentration level of about 100 pptw, while only consuming about 100 μl of the sample. The response time of the setup is about 20 s and can therefore definitely be called “online”. Moreover the method is applicable to the analysis of all substances and not limited by the permeability of a membrane.