Measurements have been made of the logarithmic decrement and period of disks of radius 3‐8.5 cm performing torsional oscillations at amplitudes in the range 0.01‐1.6 rad in fluids of kinematic viscosity 0.29‐3.06 centistokes. Both the logarithmic decrement and period were observed to depend on the amplitude of oscillation in qualitative agreement with a thory due to Rosenblat which predicts amplitude effects due to secondary flow. The amplitude effects were smaller than predicted and were dependent on the radius of the disk but were relatively insensitive to kinematic viscosity, disk thickness, or distance to the outer boundary. Below 0.2 rad the period was constant within 0.001% and below 0.3 rad the logarithmic decrement was constant within 0.3%; empirical correlations are presented for the data at higher amplitudes.