  We present the results of ISO SWS and LWS grating scans towards the embedded
Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure
rotational lines of H2 and the infrared fine structure lines of [C II], [O I],
[S I], [Si II] and [Fe II], as well as absorption bands due to H2O, CO and CO2
ice were detected toward Cep A. In S106 we detected emission lines of H2, CO, H
I, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and
Ar. S106 also shows many of the infrared PAH bands in emission. Excitation
temperatures and molecular hydrogen masses were derived from the low-lying pure
rotational levels of H2 and are 500 and 730 K and 8 and 3 x 10^{-3} solar
masses for S106 and Cep A, respectively. Since both objects are expected to
have several solar masses of H2 in their environment, we conclude that in both
cases the bulk of the H2 is cooler than a few hundred Kelvins. Excitation
temperatures and line ratios were compared with those predicted by theoretical
models for PDRs and dissociative and non-dissociative shocks. The [S I] 25.2
micron/[Si II] 34.8 micron ratio is a particularly useful shock versus PDR
discriminant and we conclude that S106 IR is dominated by PDR emission while
Cep A East has a large shock component. From an analysis of the ionic lines in
S106 we conclude that the central star must have a temperature around 37,000 K,
corresponding to a spectral type of O8. From its luminosity it is concluded
that the driving source of Cep A must also be a massive early-type star. The
absence of strong high-ionization ionic lines in its ISO spectrum shows that
Cep A has not yet created a significant H II region and must be younger than
S106, illustrating the process of the clearing of the surroundings of a massive
young star.
