Hi, using a FunctionAssigner should work. By first interpolating the expressions to the non-Mixed spaces, then using a FunctionAssigner to assign each of them to the mixed space. Demo at: https://bitbucket.org/fenics-project/dolfin/raw/ec57db53f2b13f1768214829e8b4f80fc32205cf/python/demo/undocumented/sub-function-assignment/demo_sub-function-assignment.py