The information at present available concerning isotope effects in the nuclear charge distribution in the ground state for the abundant isotopes of tin is discussed. Recent spectroscopic data are included, leading to new results for the changes in mean square radius
δ
<
r
2
> over this range of nuclei; 116–117, 0.049; 116–118, 0.124; 118–119, 0.044; 118–120, 0.112; 120–122, 0.101; 122–124, 0.091 (all in square femtometres). The error in all cases is estimated to be ± 7%. These values are on average less than half those calculated assuming incompressible nuclear matter, and for the even isotopes decrease regularly with mass number. A comparison of electronic and muonic isotope shift measurements in terms of a two-parameter Fermi distribution of nuclear charge shows that the smallness of
δ
(
r
2
) may be due to changes occurring in the surface region of the nucleus, the nucleon density in the central region remaining approximately constant. This result is consistent with data from electron scattering experiments. The contributions to
δ
(
r
2
) due to changes in nuclear deformation are evaluated.
