A process for the start-up of an ATR (autothermal reformer), the ATR comprising an ATR reactor with a burner, said process comprising the steps of heating an inert gas by a heating means, and introducing the heated inert gas in a circulation loop containing the ATR reactor; heating a mixture of inert gas and process steam by a heating means, and introducing the heated mixture of inert gas and process steam in the circulation loop containing the ATR reactor; introducing heated process steam in the circulation loop as soon as a minimum outlet threshold temperature of the ATR reactor is reached, whereby inert gas is removed from the circulation loop; heating a hydrocarbon containing process stream by a heating means, and introducing the heated hydrocarbon containing process stream into the ATR reactor, whereby a syngas stream containing carbon oxides and hydrogen is produced in the ATR reactor through steam reforming; cooling the produced syngas stream to separate water from the syngas stream as process condensate, whereby a dry syngas stream is obtained; recycling a fraction of the dry syngas stream from a downstream section of the ATR reactor to an upstream section of the ATR reactor; whereby a mixed process stream is obtained at the upstream section of the ATR reactor, the mixed process stream containing the process steam, the hydrocarbon containing process stream and the recycled dry syngas stream; introducing the mixed process stream to the burner of the ATR reactor; introducing an oxygen containing process stream to the burner of the ATR reactor as soon as a minimum hydrogen threshold concentration in the mixed stream at the upstream section of the ATR reactor is reached; whereby autoignition of the mixed process stream is obtained; and operating the ATR to produce syngas through autothermal reforming. said process comprising the steps of heating an inert gas by a heating means, and introducing the heated inert gas in a circulation loop containing the ATR reactor; heating a mixture of inert gas and process steam by a heating means, and introducing the heated mixture of inert gas and process steam in the circulation loop containing the ATR reactor; introducing heated process steam in the circulation loop as soon as a minimum outlet threshold temperature of the ATR reactor is reached, whereby inert gas is removed from the circulation loop; heating a hydrocarbon containing process stream by a heating means, and introducing the heated hydrocarbon containing process stream into the ATR reactor, whereby a syngas stream containing carbon oxides and hydrogen is produced in the ATR reactor through steam reforming; cooling the produced syngas stream to separate water from the syngas stream as process condensate, whereby a dry syngas stream is obtained; recycling a fraction of the dry syngas stream from a downstream section of the ATR reactor to an upstream section of the ATR reactor; whereby a mixed process stream is obtained at the upstream section of the ATR reactor, the mixed process stream containing the process steam, the hydrocarbon containing process stream and the recycled dry syngas stream; introducing the mixed process stream to the burner of the ATR reactor; introducing an oxygen containing process stream to the burner of the ATR reactor as soon as a minimum hydrogen threshold concentration in the mixed stream at the upstream section of the ATR reactor is reached; whereby autoignition of the mixed process stream is obtained; and operating the ATR to produce syngas through autothermal reforming.