These methods require the analysis of the vapour and the liquid phases. This can be done either on withdrawn samples or directly in situ. Sampling is a challenging job, because the samples must be representative for the equilibrium phases and the withdrawal of samples must not disturb the equilibria studied, e.g. the total pressure must remain constant during sampling. Unfortunately, these conditions are rarely rigorously fulfilled. In situ analyses are very attractive but they can be applied in very few cases only.