As interpolation and spline-interpolation usually generate awes which do not preserve monotonicity or convexity, some problems in natural sciences and engineering cannot: be solved by these methods in a satisfactory way. These difficulties are shown by means of a subject of saturation in adsorption. Two algorithms will be presented which produce co-monotone C-1- and C-2-curves. They are constructed with aid of the Bernstein-operator. It is shown that co-monotone C-1-splines solve the above problem. A disadvantage of C-2-splines is discussed. The reasons for the properties of this kind of interpolation are given.