Looking at coal conversion processes coal-oil slurries are exposed to high and changeable process temperatures and pressures. In additional to that process gases can affect the system. Vaporization of parts of the suspension liquid, dissolvation of gas components in the suspension as well as thermal forced swelling and dissolvation processes lead to viscosity changes. Experimental determination of viscosity data at process conditions is very difficult. Therefore the results of changeable process conditions were calculated by the help of information on the nature of system. The calculations include the influence of physical state changes on the basis of equilibrium thermodynamics. As a result of investigations the dimensions of viscosity changes of brown-coal suspensions could be derived at several influences.