A method for determining a carbon content value of a hydrocarbon-containing mixture. At least one composition-dependent bulk property of the hydrocarbon-containing mixture is measured and optionally at least one non-hydrocarbon component concentration is measured with the resulting measurements used in a carbon content correlation for calculating the carbon content of the hydrocarbon-containing mixture. The carbon content may be used in a hydrogen and/or synthesis gas production process for calculating a target flow rate of steam to be combined with the hydrocarbon-containing mixture to form a mixed feed having a target steam-to-carbon ratio.