Summary: Instabilities of a viscous fluid between two fast but differentially rotating concentric spheres, the so-called spherical Couette flow, with a fixed radius ratio of \(\eta=r_i/r_o=1/3\) are studied, where \(r_i\) is the inner and \(r_o\) the outer radius of the spherical shell. Of particular interest is the difference between cases where the Rossby number \(Ro=(\Omega_i-\Omega_o)/\Omega_o>0\) and cases with \(Ro<0\), where \(\Omega_i\) and \(\Omega_o\) are the inner- and outer-sphere angular velocities. The basic state in both situations is an axisymmetric shear flow with a Stewartson layer situated on the tangent cylinder. The tangent cylinder is given by a cylinder that touches the equator of the inner sphere with an axis parallel to the axis of rotation. The experimental results presented fully confirm earlier numerical results obtained by R. Hollerbach [ibid. 492, 289-302 (2003; Zbl 1063.76558)] showing that for \(Ro>0\) a progression to higher azimuthal wavenumbers \(m\) can be seen as the rotation rate \(\Omega_0\) increases, but \(Ro<0\) gives \(m=1\) over a large range of rotation rates. It is further found that in the former case the modes have spiral structures radiating away from Stewartson layer towards the outer shell whereas for \(Ro<0\) the modes are trapped in the vicinity of the Stewartson layer. Further, the mean flow excited by inertial mode self-interaction and its correlation with the mode’s amplitudes are investigated. The scaling of the critical \(Ro\) with Ekman number \(E=\nu/(\Omega_o\,d^2)\), where \(\nu\) is the kinematic viscosity and \(d\) the gap width, is well within the bounds that have been established in a number of experimental studies using cylindrical geometries and numerical studies using spherical cavities. However, the present work is the first that experimentally examines Stewartson-layer instabilities as a function of the sign of \(Ro\) for the true spherical-shell geometry.