The use of isothermal thermogravimetry is investigated for combined measurements of activity coefficients and diffusion coefficients in binary mixtures, where the vapor pressure is dominated by one compound. The coupling of evaporation with gas phase and condensed phase diffusion leads to an unsteady mass loss, which depends on the activity coefficient and both, gas phase and condensed phase, diffusion coefficients, as is explained. This can be used to measure two of these properties, if the remaining parameters are known. This concept is evaluated for the ferrocene/n-tetracosane system. The former is a flame-retardant, and the latter was regarded as a substitute for a polymer, with simpler behavior. Between 341 and 399 K, the activity coefficient for ferrocene is measured to drop from above 3 to values near 1, while the diffusion coefficients are weakly temperature dependent, with values around 2 x 10(-9)/m(2)s(-1). In addition, the vapor pressures of the two pure substances were re-evaluated for this temperature range. This relatively simple method seems to be useful for the determination of such properties of other flame-retardant-related systems.