With a controlled environment electron microscope it is possible to remove the effect of another atmosphere, such air, to which the sample may have become exposed; to study in considerable detail the continuous development of a series of microstructures in a dynamic process, to identify important intermediate phases and defects in them which may only be metastable under specific reaction conditions of atmosphere and temperature, and to establish whether they are the origin, or merely the consequence, of the reaction. In this paper we report the extensive modification of a high resolution transmission electron microscope (Philips CM30T HRTEM) to add facilities for dynamic in-situ gas-solid reaction studies in a controlled atmosphere of gas or vapor at pressures of 0–50mbar which replaces the regular high vacuum TEM environment. The new environmental cell (ECELL) capability is combined with the original 0.23nm TEM resolution, STEM (BF/ADF) imaging and chemical and crystallographic analyses. Regular specimen holders are used; including hot stages capable of temperatures >1000°C. The full TEM resolution has been demonstrated at temperatures above 700°C and with limited (mbar) amounts of gas flowing through the integral 4-aperture differentially pumped ECELL (and with the additional pumps for the system operating). The ECELL system can be used to preserve, or if necessary to recreate, a continuous chain of evidence in a chemical reaction process.