Most acid core flow testing utilizes short (<3") core plugs. In this study flow tests were run using both short core plugs and long(>8") core plugs. All plugs were from a producing formation in the Gulf of Mexico. Both long and short cores were damaged with a fluid loss control pill and then treated with various acid systems. When treated with HC1 or HC1:HF the short core plugs showed an increase in permeability over the length of the core. Long cores treated with HC1:HF showed an increase in permeability in the first 2 inches of the plug but a decrease in permeability farther down the plug. Analysis of the spent HC1:HF indicated that the loss of permeability was caused by precipitation products from the HF/clay reaction. Observations made during these tests indicate that relying on short core flow tests for selection of a matrix treatment fluid can result in a damaging treatment. Long core tests provide a better tool for selection matrix treatment fluids.