The invention provides a method for determining the dew point of a vaporised hydrocarbon feedstock, comprising heating a hydrocarbon feedstock and a hydrocarbon diluent to a first temperature and maintaining the feedstock and diluent at that temperature until all hydrocarbon feedstock is vaporized and a homogeneous mixture is obtained; passing a first flow of the mixture through a first zone maintained at a second temperature, which is lower than the first temperature; halting the first flow; passing a sweep gas through the first zone and providing the sweep gas to a detector for detecting hydrocarbons; decreasing the temperature of the first zone, wherein steps (c) to (f) are repeated at least until a hydrocarbon presence is detected in the sweep gas.The invention further provides a method for measuring the dew point of a steam cracker feed and a system for determining the dew point of a vaporised hydrocarbon feedstock.