In order to determine whether there is substantial infrared
 emission from the ionized component of the interstellar medium,
 we propose to observe a field, already mapped in the H-alpha and
 HI lines at 3 arcmin resolution, using ISOPHOT. To date, the
 infrared emission from the diffuse, ionized component of the
 interstellar medium at high galactic latitude has never been
 detected. Such emission, if present, would have a substantial
 impact on the infrared background inferred from IRAS or
 COBE/DIRBE analyses, which use the atomic gas alone as a tracer
 of the interstellar medium. The field can be covered in a 13 x
 19 raster of pointings. Two wavebands (100 and 160 microns) will
 be used, so that the color of the infrared emission from the
 atomic and ionized media (if detected) can be separately
 determined.