  This article is dedicated to the use the MICROSCOPE mission's data to test
chameleon theory of gravity. We take advantage of the technical sessions aimed
to characterize the electrostatic stiffness of MICROSCOPE's instrument
intrinsic to its capacitive measurement system. Any discrepancy between the
expected and measured stiffness may result from unaccounted-for contributors,
i.e. extra-forces. This work considers the case of chameleon gravity as a
possible contributor. It was previously shown that in situations similar to
these measurement sessions, a chameleon fifth force appears and acts as a
stiffness for small displacements. The magnitude of this new component of the
stiffness is computed over the chameleon's parameter space. It allows us to
derive constraints by excluding any force inconsistent with the MICROSCOPE
data. As expected --since MICROSCOPE was not designed for the purpose of such
an analysis--, these new bounds are not competitive with state-of-the-art
constraints, but they could be improved by a better estimation of all effects
at play in these sessions. Hence our work illustrates this novel technique as a
new way of constraining fifth forces.
