A process is provided for determining retorting conditions in an in situ oil shale retort. In the process, the thickness of the hot shale zone is determined by monitoring the off gases in response to changing the feed conditions, such as the temperature or flow rate, of the feed gas. The location and depth of the hot shale zone can be determined by monitoring the response time and temperature of the off gases when the proportion of steam and air in the feed gas is changed. The depth of the mineral decomposition zone can be monitored by monitoring the amount of carbon dioxide produced when the air content of the feed gas is substantially reduced. The depth of the kerogen decomposition zone can be determined by monitoring the amount of hydrogen saturated gases produced when the inflow of air in the feed gas is stopped.