The invention relates to a method of studying a sample in the active atmosphere in an Environmental Transmission Electron Microscope (ETEM). Such a study is used to study a reaction (chemical or physical) of a sample with a gas. Of special interest is the chemical reactions of a gas and a catalyst, as well as the physical change (phase change) of, for example, the growth of a whisker from the gas phase growing on a solid. Prior art studies involve introducing the sample on a side-entry sample holder in the sample chamber of an ETEM, heating the sample to a required temperature, wait for the sample to settle to a drift-free position and then expose the sample to a pressure of, for example, 10 mbar of heated, reactive gas. Because the temperature distribution of the sample holder is a function of the temperature and pressure of gas, the temperature distribution over the sample holder will slightly change when exposing the sample holder to the gas, as a result of which the sample will drift. To avoid, or at least minimize, the drift, the invention involves the exposure to inert gas at a desired temperature before exchanging the inert gas to the active gas. The invention is also of applicable to optical, X-ray or scanning probe microscopy.