We propose to study the physical conditions of the gas in the
 innermost region of the collapsing envelopes surrounding Young
 Stellar Objects. In preparation of the ISO mission, we developed
 a theoretical model (Ceccarelli, Hollenbach and Tielens, 1994)
 for the predictions of the far infrared line spectrum from a
 collapsing envelope onto a central object: the model predicts
 that many water lines and the OI(63um) will be easily detectable
 by the ISO spectrometers, while CO lines are weaker. Through a
 careful study of the brightest lines, we selected seven lines
 (five water lines, OI(63um) line and one CO line) which probethe
 temperature and density of the gas across the envelope. Moreover,
 observations of these lines allow to give a direct measure of the
 water abundance, the mass accretion rate and the presence of a hot

 component due to a hot circumstellar disk. The three lines in the
 SWS range come from regions whose infalling velocity can be
 resolved by the Fabry-Perot. The remaining four lines in the LWS
 range lie on such strong continua that high resolution observations
 are needed to detect them. We hence request the SWS07 and LWS04
 modes to observe the proposed lines. We notice that water lines are
 only observable from a space borne platform and ISO provides unique

 opportunity to detect them. Finally comparison of the predicted
 line emission from the infalling envelope (our model) with the model
 by Neufeld and Melnick (1987), that describes water emission from
 shocked gas (outflow), shows that the two regimes are easily
 distinguishable.