In this paper, topological
techniques are used to establish existence results for some boundary value problems
arising in diffusion theory. Questions of uniqueness are also treated. Our topological
arguments are based on the topological transversality theorem rather than the
Leray-Schauder theory. An important feature of our approach is that some of the
results obtained cannot be deduced by a direct application of the latter theory.
Further applications of topological transversality to differential equations will be
given in forthcoming parts of the paper.