As the microwave background (MWB) photons travel towards us
 through rich clusters of galaxies, they are scattered to higher
 energies by the hot gas in the clusters. The resulting spectral
 distortion is called the Sunyaev-Zeldovich (SZ) effect and
 consists of a decrement in the MWB intensity in the direction of
 the cluster relative to the general background at wavelengths
 >1.4mm and an increment at wavelengths <1.4mm. The cosmological
 importance of the SZ effect is that, when coupled with X-ray
 observations of the hot scattering gas, it is possible to deduce
 the scale size of the universe, ie the Hubble constant, H_0.
 With the advent of the imaging sub-millimetre array, SCUBA, on
 the James Clerk Maxwell Telescope (JCMT) it will soon be
 possible to make observations of the SZ effect in the increment
 regime, where the spectral distortion should be large. However
 there have recently been some indications that clusters of
 galaxies may contain large amounts (10^9 solar massess) of very
 cool dust (temperatures of 10-30 K). Such dust would dominate
 the cluster emission at 200 microns and, although largely
 undetectable by IRAS, could also radiate noticeably in the
 sub-millimetre waveband and so could confuse observations of the
 SZ increment. We therefore propose ISOPHOT 200 observations,
 through 3 filters, to search for possible dust emission from a
 sample of SZ candidate clusters, and to measure its temperature,
 in order to determine whether dust may confuse future
 determinations of H_0.