Start-up transition process of a fuel cell system operation state, in which system each fuel cell comprises an anode side, a cathode side, and an electrolyte between the anode side and the cathode side, and in the process is performed a recirculation in a recirculation loop containing at least the fuel cell anodes and a catalytic element upstream of the fuel cells, characterized by, that in the start-up transition process is performed - utilization of predefined first and second temperature limit for the fuel cells, specifying a low temperature operating state of cells below the first limit, at which presence of carbonaceous species at the cells is precluded, a transition temperature range of cells above the first and below the second limit, at which is initiated fuel flow supply to the fuel cell system in a mixture with air, combined with anode tail gas recirculated at a recirculation rate over 70%, the recirculation being at least partly supported by ejector facilitated repressurization operations, and an intermediate temperature operating state of the cells above the second temperature limit, at which free oxygen at the anodes is precluded, - bringing temperature of the cells in the system to transition temperature range to facilitate transitions between low and intermediate temperature operating states through the transition temperature range, - facilitating and safeguarding reaction between fuel and free oxygen supplied at the catalytic element whenever fuel is supplied, and - control of oxygen to carbon ratio of fluid at the anode sides of cells based on predefined temperature dependent boundary values by adjustment of air to fuel ratio λ of feed stocks above 0.55*λ at transition temperature range and controlling the λ of the feed stocks based on temperature information in the intermediate temperature range and further reducing λ of the feed stocks when loading is applied to the fuel cells to compensate for oxygen influx through the cells to the anode sides of the cells and to control fuel utilization. - utilization of predefined first and second temperature limit for the fuel cells, specifying a low temperature operating state of cells below the first limit, at which presence of carbonaceous species at the cells is precluded, a transition temperature range of cells above the first and below the second limit, at which is initiated fuel flow supply to the fuel cell system in a mixture with air, combined with anode tail gas recirculated at a recirculation rate over 70%, the recirculation being at least partly supported by ejector facilitated repressurization operations, and an intermediate temperature operating state of the cells above the second temperature limit, at which free oxygen at the anodes is precluded, - bringing temperature of the cells in the system to transition temperature range to facilitate transitions between low and intermediate temperature operating states through the transition temperature range, - facilitating and safeguarding reaction between fuel and free oxygen supplied at the catalytic element whenever fuel is supplied, and - control of oxygen to carbon ratio of fluid at the anode sides of cells based on predefined temperature dependent boundary values by adjustment of air to fuel ratio λ of feed stocks above 0.55*λ at transition temperature range and controlling the λ of the feed stocks based on temperature information in the intermediate temperature range and further reducing λ of the feed stocks when loading is applied to the fuel cells to compensate for oxygen influx through the cells to the anode sides of the cells and to control fuel utilization.