A new wide-annulus Couette-type viscometer/rheometer has been designed, fabricated, and tested with the objective of measuring the shear flow properties of silicate melts and magmatic suspensions containing vapor bubbles and/or solid particles at10−4 GPaand temperatures from 600 to 1500 °C. The goal of this development has been to advance the precision of high-temperature multiphase silicate rheometry in the viscosity regime of104–1010 Pa s,and to attempt experiments on multiphase magmatic mixtures that have heretofore been difficult to perform in the concentric cylinder geometry. With this instrument, fluids with viscosities in the design viscosity range may be tested at shear rates from10−5to 1 s−1, employing a series of interchangeable torque transducers capable of measurements from 0.003 to 3 N m, preserving precision of the stress measurement across the entire dynamic span. A microprocessor-controlled motor drive enables precisely specified rotational motions (position and/or speed of the sample container) for accurate constant-shear-rate or finite-strain experiments. Use of high-purity cast ceramic components rather than platinum alloy for the rotating sample container (cup) and fixed cylindrical bob permit sectioning of the sample following an experiment to inspect and analyze textural characteristics of the magmatic suspensions. Computerized data acquisition is accomplished via a standard microcomputer interface and commercially available software system that allows data to be recorded, visualized, and statistically modeled as an experiment progresses. The instrument has been tested using NIST (NBS) reference material 717 (a borosilicate glass) at viscosities from104to109 Pa s.Value of the standard viscosity has been recovered within the published precision (0.03 log units) at temperatures between 650 and 850 °C. ©1998 American Institute of Physics.