An Environmental Transmission Electron Microscope suffers from gas-induced resolution deterioration. It is found that this deterioration was not a function of the current density on the sample, but of the total current of the beam of electrons. Inventors conclude that the deterioration is due to ionization of gas in the sample chamber of the ETEM, and propose to use an electric field in the sample chamber to remove the ionized gas, thereby diminishing the gas-induced resolution deterioration. The electric field need not be a strong field, and can be caused by, for example, biasing the sample 114 with respect to the sample chamber 138. A bias voltage of 100 V applied via voltage source 144 is sufficient for a marked improvement the gas-induced resolution deterioration. Polarization is not important. Alternatively an electric field perpendicular to the optical axis 104 can be used, for example by placing an electrically biased wire or gauze 154 off-axis in the sample chamber.