An automatic reactor system for catalytic microactivity studies, characterised in that the system comprises: - a hot box, comprising inside therein: ▪ a preheater for gases and a preheater-evaporator for liquid reactants, connected to ▪ a bypass that is a six-way valve, configured to select a process path for a mix of the gases and liquid reactants, and connected to ▪ a reactor positioned inside a furnace and having a catalyst disposed therein, being configured to obtain liquid and gas products; - various different mass flow regulators (1, 1', 1") located outside the hot box, and configured to supply a known controlled stream of gases to said preheater for gases, - a HPLC pump located outside the hot box and configured to supply liquid reactants from a container to said preheater-evaporator for liquid reactants, - a liquid-gas separator, disposed outside the hot box and being configured to perform a cold condensation under pressure and to permit a continuous separation of the liquid products and the gas products and their evacuation thereof by a liquids output line and a gases output line respectively; wherein the liquid-gas separator has a liquids accumulation volume of 0,1 - 0,5 cm 3 and a minimum dead volume, and comprises a high resolution capacitive level sensor with a precision of ± 0,1 cm 3; wherein the liquid-gas separator is connected to a level controller configured to control the level in said liquid-gas separator and to control a servo-positioned microregulation valve located in the liquids output line; and to a pressure control system consisting of a servo-positioned microregulation valve located in the gases output line and inside the hot box, to control the reactor pressure; and wherein the liquid-gas separator is automated and computerized, having local control and remote control on-line, based on digital communications with a distributed control structure, a safety system being integrated into a microprocessor independent of the computerized control structure. - a hot box, comprising inside therein: ▪ a preheater for gases and a preheater-evaporator for liquid reactants, connected to ▪ a bypass that is a six-way valve, configured to select a process path for a mix of the gases and liquid reactants, and connected to ▪ a reactor positioned inside a furnace and having a catalyst disposed therein, being configured to obtain liquid and gas products; ▪ a preheater for gases and a preheater-evaporator for liquid reactants, connected to ▪ a bypass that is a six-way valve, configured to select a process path for a mix of the gases and liquid reactants, and connected to ▪ a reactor positioned inside a furnace and having a catalyst disposed therein, being configured to obtain liquid and gas products; - various different mass flow regulators (1, 1', 1") located outside the hot box, and configured to supply a known controlled stream of gases to said preheater for gases, - a HPLC pump located outside the hot box and configured to supply liquid reactants from a container to said preheater-evaporator for liquid reactants, - a liquid-gas separator, disposed outside the hot box and being configured to perform a cold condensation under pressure and to permit a continuous separation of the liquid products and the gas products and their evacuation thereof by a liquids output line and a gases output line respectively;