  Aims. We observe occultations of WASP-24b to measure brightness temperatures
and to determine whether or not its atmosphere exhibits a thermal inversion
(stratosphere). Methods. We observed occultations of WASP-24b at 3.6 and 4.5
{\mu}m using the Spitzer Space Telescope. It has been suggested that there is a
correlation between stellar activity and the presence of inversions, so we
analysed existing HARPS spectra in order to calculate log R'HK for WASP-24 and
thus determine whether or not the star is chromospherically active. We also
observed a transit of WASP-24b in the Str\"{o}mgren u and y bands, with the
CAHA 2.2-m telescope. Results. We measure occultation depths of 0.159 \pm 0.013
per cent at 3.6 {\mu}m and 0.202 \pm 0.018 per cent at 4.5 {\mu}m. The
corresponding planetary brightness temperatures are 1974 \pm 71 K and 1944 \pm
85 K respectively. Atmosphere models with and without a thermal inversion fit
the data equally well; we are unable to constrain the presence of an inversion
without additional occultation measurements in the near-IR. We find log R'HK =
-4.98 \pm 0.12, indicating that WASP-24 is not a chromospherically active star.
Our global analysis of new and previously-published data has refined the system
parameters, and we find no evidence that the orbit of WASP-24b is non-circular.
Conclusions. These results emphasise the importance of complementing Spitzer
measurements with observations at shorter wavelengths to gain a full
understanding of hot Jupiter atmospheres.
