We propose to obtain linear scans of the thermal emission from dust in
 the envelopes of 7 to 8 proto-planetary nebulae (PPNe) and planetary
 nebulae (PNe)  at 95um and 170um using the ISOPHOT FIR arrays, C100
 and C200 respectively. The two goals of these observations is (1) to
 determine the history of mass loss as imprinted on the circumstellar
 dust envelopes in these PPNe and PNe, and (2) to measure the total
 mass of the circumstellar dust from which we can infer a lower limit
 to the initial mass of the progenitor main sequence star. In particular,
 the radial profiles of dust density derived from these observations will
 test current theories of episodic mass loss and accelerated mass loss.
 Understanding this mass loss process is a key problem within
 astrophysics because most of interstellar dust and gas originates
 from these stellar sources.  ISO provides the necessary sensitivity
 and longer wavelength coverage to detect the ~35K thermal dust halos
 surrounding PPNe and PNe.