The lateral diffusion of the spectroscopic probe-quencher pair N-(1-pyrenesulfonyl)dipalmitoyl-L-alpha- phosphatidylethanolamine triethylammonium salt (pyrene-DPPE) and 4-(N,N-dimethyl-N-hexadecylammonio)-2,2,6,6-tetramethylpiperidine-1-oxyl, iodide (CAT-16) has been measured in Langmuir-Blodgett (LB) films in contact with aqueous solution. The mutual lateral diffusion coefficients were determined to be in the range (6-8) x 10(-9) cm(2) s(-1) by analyzing the time-resolved fluorescence decay of the excited pyrene-DPPE in the presence of CAT-16. A comparison of diffusion in "wet" LB films with "dry" films and air-water monolayers shows the diffusion in the "wet" film is considerably faster than in the "dry" film, but approximately half to 1 order of magnitude slower than in an air-water monolayer. In addition, it is shown that the state of dispersion of pyrene-DPPE in the deposited dim is highly sensitive to the number of LB layers deposited and the composition of the film.