A system for surface characterization of a porous solid or powder sample using flowing gas, said system comprising: a first mass flow controller configured to deliver a controllable mass flow of a carrier gas; a second mass flow controller configured to deliver a controllable mass flow of an adsorptive gas; a mixer having an input coupled to the first and second mass flow controllers and configured to deliver a mixture of the carrier gas and the adsorptive gas to at least a first measurement channel; a first flow control device coupled to the mixer; a first sample cell disposed within the first measurement channel, the first sample cell configured to contain a first amount of the sample and flow the mixture over or through the first amount of the sample; a first chiller configured to selectively cool the first sample cell; a first concentration detector coupled downstream of the first sample cell and configured to provide a signal indicative of adsorptive gas concentration in the mixture downstream of the first sample cell; and wherein the system is characterised in that it comprises: a controller coupled to the first and second mass flow controllers and the first concentration detector, the controller configured to control the first and second mass flow controllers in response to respective first and second output signals from the first and second mass flow controllers to deliver a target concentration of the adsorptive gas in the mixture to the first sample cell and to determine an amount of the adsorptive gas adsorbed and desorbed by the sample based on signals from the first concentration detector and the first and second output signals. a first mass flow controller configured to deliver a controllable mass flow of a carrier gas; a second mass flow controller configured to deliver a controllable mass flow of an adsorptive gas; a mixer having an input coupled to the first and second mass flow controllers and configured to deliver a mixture of the carrier gas and the adsorptive gas to at least a first measurement channel; a first flow control device coupled to the mixer; a first sample cell disposed within the first measurement channel, the first sample cell configured to contain a first amount of the sample and flow the mixture over or through the first amount of the sample; a first chiller configured to selectively cool the first sample cell; a first concentration detector coupled downstream of the first sample cell and configured to provide a signal indicative of adsorptive gas concentration in the mixture downstream of the first sample cell; and wherein the system is characterised in that it comprises: a controller coupled to the first and second mass flow controllers and the first concentration detector, the controller configured to control the first and second mass flow controllers in response to respective first and second output signals from the first and second mass flow controllers to deliver a target concentration of the adsorptive gas in the mixture to the first sample cell and to determine an amount of the adsorptive gas adsorbed and desorbed by the sample based on signals from the first concentration detector and the first and second output signals.