A scheme based on the friction theory (f-theory) is introduced for the viscosity prediction of mixtures composed of hydrogen and natural gas (hythane). In the f-theory the viscosity is separated into a dilute gas viscosity and a friction contribution term. The mixture friction coefficients are estimated with mixing rules based on pure-component friction coefficients. Because hythane mainly contains hydrogen and methane, the pure friction coefficients of these components are obtained with f-theory models directly fitted to these two components, while the friction coefficients of the other components are obtained with a general f-theory model. For the dilute gas viscosity term, the simple mixing rule of Wilke is capable of an accurate estimation of the dilute gas viscosity. Using this f-theory scheme in conjunction with the Peng-Robinson and Soave-Redlich-Kwong EOSs, the viscosities of four hythane mixtures have been predicted within or close to experimental uncertainty (+/-1.0%), which is satisfactory for most industrial applications. This scheme is of a predictive character, because only properties and parameters of the pure compounds are required. This work further shows the application of the f-theory for viscosity predictions and its application to industrial processes.