  We show that LHC experiments might well be able to determine all the
parameters required for a prediction of the present density of thermal LSP
relics from the Big Bang era. If the LSP is an almost pure bino we usually only
need to determine its mass and the mass of the SU(2) singlet sleptons. This
information can be obtained by reconstructing the cascade $\tilde{q}_L \to
\tilde{\chi}_2^0 q \to \tilde{\ell}_R \ell q \to \tilde{\chi}_1^0 \ell^+ \ell^-
q$. The only requirement is that $m_{\tilde{\ell}_R} < m_{\tilde{\chi}_2^0}$,
which is true for most of the cosmologically interesting parameter space. If
the LSP has a significant higgsino component, its predicted thermal relic
density is smaller than for an equal--mass bino. We show that in this case
squark decays also produce significant numbers of $\tilde{\chi}_4^0$ and
$\tilde{\chi}_2^\pm$. Reconstructing the corresponding decay cascades then
allows to determine the higgsino component of the LSP.
