Two detonation tubes were used to Study detonation propagation in concentration gradients. Each tube, 22 x 10 mm and 50 nun diameter in cross section, respectively, was fitted with slide valves that with the tubes mounted vertically allowed known concentration gradients to form by diffusion. The peak-transmitted shock strength when a detonation is incident on an abrupt planar interface with an inert gas was found to agree reasonably well with the simple Paterson-Glass model. A significant improvement was found in the predicted values when allowance was made for the Taylor expansion and energy and momentum losses to the tube walls. Under certain conditions a secondary pressure pulse was observed behind the transmitted shock; the origin of this pulse is the explosion of the unreacted gas between the transmitted shock and the unburned gas. When incident on gradients of fuel concentration, the transmitted detonation wave velocity and cell size adjusted rapidly, to values appropriate to the local gas composition and wall boundary layer losses. Finally, it was shown that the mechanism of detonation initiation in a weaker mixture by an incident detonation propagated across a concentration gradient differs in one important respect from that in a homogeneous mixture in that a secondary shock is formed as an intermediate stage.