The chronoamperometric response at an ultramicrodisk electrode (diameter = 25 mu m) inserted into polyacrylamide (PAAM) gel films containing reducible electroactive species [Ru(NH3)(6)(3+) and methylene blue (MB)I was used to determine the diffusion coefficients (D) and concentrations (C) of these species using a previously proposed treatment of the ratio of transient and steady-state currents. The diffusion coefficients obtained in film were slightly smaller (75-95%) than the values for the same species in solution, demonstrating that the film environment is mainly an aqueous one contained within large polymer pores. The calculated concentrations were in good agreement with those used to cast the films. Complexes of MB with DNA were also examined in solution and in a PAAM him. The diffusion coefficients in both cases were significantly smaller (similar to 23%) than those of MB alone. The results indicate that, with some limitations, the chronoamperometric response of films probed with an SECM tip can be used to determine both D and C with knowledge of only the ultramicrodisk radius.