This work presents a spectrum of possible derivative responses in wells producing commingled reservoirs. Three issues pertaining to commingled reservoir buildup behavior are studied: (1) derivative responses for commingled systems, with emphasis on unequal layer radii; (2) pressure distributions in the layers with a view toward understanding backflow, particularly the origin of rate reversals during buildup; and (3) an application of Peaceman's probe radius formula to obtain pressure distributions in each layer at shut-in. Several approaches are developed from Issue 1 to estimate the areal extent of the smallest layer. Some general information that should be useful, or at least pertinent, to the design of production logging surveys is derived from Issue 2. Issue 3 also has application in miscible flooding for the location of areas below minimum miscibility pressure (MMP).