  Observations of HDO are an important complement for studies of water, because
they give strong constraints on the formation processes -- grain surfaces
versus energetic process in the gas phase, e.g. in shocks. The HIFI
observations of multiple transitions of HDO in Sgr~B2(M) presented here allow
the determination of the HDO abundance throughout the envelope, which has not
been possible before with ground-based observations only. The abundance
structure has been modeled with the spherical Monte Carlo radiative transfer
code RATRAN, which also takes radiative pumping by continuum emission from dust
into account. The modeling reveals that the abundance of HDO rises steeply with
temperature from a low abundance ($2.5\times 10^{-11}$) in the outer envelope
at temperatures below 100~K through a medium abundance ($1.5\times 10^{-9}$) in
the inner envelope/outer core, at temperatures between 100 and 200~K, and
finally a high abundance ($3.5\times 10^{-9}$) at temperatures above 200~K in
the hot core.
