Central question in our analysis is if voluntarily organised Professions, i.e. self-employed Liberal Professions (Freiberufler) in professional organisations, yield a higher income than those not associated to any professional organisation. To answer this question four waves from the German Socio-Economic Panel Survey Study (GSOEP) are used. Descriptive statistics show a positive link of gross monthly income and those who are voluntarily associated in professional organisations. Because gross monthly income is influenced by the amount of actual hours worked, gross hourly income is the central magnitude of interest. The aforementioned positive link is widely confirmed by descriptive statistics of corresponding gross hourly income. To measure causal effects through voluntary membership in professional organisations a fixed-effects-model in consideration of unobserved and time constant heterogeneity is estimated. We find a negative premium in gross hourly income for voluntarily organised Professions which varies between -7,67 und -9,15 %.