diff --git a/doc/source/users/user_manual/irf.rst b/doc/source/users/user_manual/irf.rst
index a269b49..8fa2ae5 100755
--- a/doc/source/users/user_manual/irf.rst
+++ b/doc/source/users/user_manual/irf.rst
@@ -10,3 +10,4 @@ Instrument Response Functions
    irf_cta
    irf_lat
    irf_comptel
+   irf_spi
diff --git a/doc/source/users/user_manual/irf_spi.rst b/doc/source/users/user_manual/irf_spi.rst
new file mode 100755
index 0000000..cd4f0e5
--- /dev/null
+++ b/doc/source/users/user_manual/irf_spi.rst
@@ -0,0 +1,27 @@
+.. _um_irf_spi:
+
+SPI response functions
+----------------------
+
+Formulation
+~~~~~~~~~~~
+
+A SPI event is characterised by an instrument direction, spanned by the telescope
+pointing direction :math:`(\alpha_{\rm spix}, \delta_{\rm spix})` and the
+pseudo-detector identifier ``DETID``, and the measured energy :math:`E'`.
+The SPI response is stored in the form of FITS files that can be downloaded
+using
+
+.. code-block:: bash
+
+   $ rsync -Lzrtv isdcarc.unige.ch::arc/FTP/arc_distr/ic_tree/prod/ $REP_BASE_PROD
+
+where ``REP_BASE_PROD`` is the target repository where all SPI data will
+reside. Each SPI response FITS file contains the response functions for a
+given true energy :math:`E`. The response is presented in telescope coordinates,
+and gives for a given pseudo-detector identifier and true energy the effective
+area as a function of sky direction :math:`(\alpha, \delta)` with respect to
+the SPI telescope X axis
+
+.. math::
+   R(p',E',t'|p,E,t) = A_{\rm eff}(\alpha, \delta |{\tt DETID}, E)