The influence of the external heavy-atom effect (HAE) on the fluorescence properties of C(70) and a C(70) methano monoadduct is determined. For this purpose the photophysics of these [70]fullerenes is studied in polystyrene (PS) and in a related heavy-atom polymer, poly(4-bromostyrene) (PBS). In the absence of HAE (PS matrix) both fullerenes display a strong thermally activated delayed fluorescence (DF) that is more pronounced in the case of C(70). In the presence of HAE (PBS matrix) both prompt (PF) and DF intensities decrease significantly, the same happening to the delayed fluorescence lifetimes. The relative fluorescence intensities (DF intensity/PF intensity) for each fullerene are on the other hand surprisingly similar to the respective ones in PS for the full experimental temperature range. The HAE is responsible for a significant increase of the S(1)→T(1) and S(1)←T(1) intersystem crossing (ISC) rates, and of the T(1)→S(0) radiative rate. In particular, the HAE on a S(1)←T(1) ISC rate is reported here for the first time. The overall substantial insensitivity of relative fluorescence intensities to HAE is explained by a compensation effect: As the S(1)→T(1) and S(1)←T(1) ISC rates on the one hand, and the T(1)→S(0) radiative rate on the other hand work in opposition with respect to DF, a near cancellation of effects occurs.