Optical sensor unit for transcutaneously measuring blood gas concentration, comprising: - a first unit comprising - sensor means , including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer ; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer , which optical response depends on the gas concentration; - first thermal insulation means at least partially surrounding said sensor means , wherein the first thermal insulation means at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means at least partially surrounding the first thermal insulation means ; and - a second unit connectable and detachable from the first unit, wherein the second unit comprises: - at least one first heating device providing heat to the first heat conducting means and - a first temperature sensor for measuring the temperature of the first heating device ; characterized in that the first heat conducting means comprise a second temperature sensor provided to measure a temperature of the first heat conducting means for controlling applied heat to the skin. - a first unit comprising - sensor means , including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer ; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer , which optical response depends on the gas concentration; - first thermal insulation means at least partially surrounding said sensor means , wherein the first thermal insulation means at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means at least partially surrounding the first thermal insulation means ; and - sensor means , including at least one sensing layer adapted to be irradiated with a predetermined radiation and at least one gas-permeable layer adjacent to one side of the at least one sensing layer and adapted to pass gas, which concentration is to measured, through the gas-permeable layer towards the at least one sensing layer ; wherein the optical sensor unit is adapted to measure an optical response of the at least one sensing layer , which optical response depends on the gas concentration; - first thermal insulation means at least partially surrounding said sensor means , wherein the first thermal insulation means at least partially surrounds lateral sides of the at least one sensing layer and - first heat conducting means at least partially surrounding the first thermal insulation means ; and - a second unit connectable and detachable from the first unit, wherein the second unit comprises: - at least one first heating device providing heat to the first heat conducting means and - a first temperature sensor for measuring the temperature of the first heating device ; - at least one first heating device providing heat to the first heat conducting means and - a first temperature sensor for measuring the temperature of the first heating device ; <EOS>