This paper pertains to the direct adaptive Proportional-Integral (PI) control of nonlinear chemical processes. A simple yet effective PI controller parameter tuning algorithm, which has different structures separated by a defined zero-level set of process output errors, is derived. This algorithm enables a PI controller to control chemical processes in an interactive and autonomous way by simply observing the process output errors. To show the stability of the resultant PI control system, a rigorous analysis involving the use of a Lyapunov approach is presented. The effectiveness and applicability of the proposed direct adaptive PI control schemes are demonstrated by considering the control of a nonlinear continuous stirred tank reactor (CSTR) in the presence of a plant/model mismatch, an unmeasured disturbance, and parameter variations. Comparisons with several conventional PI controllers and an indirect adaptive Proportional-Integral-Derivative (PID) controller are performed for performance evaluation. Simulation results indicate that the proposed PI control scheme in associated with the derived parameter tuning algorithm is promising and that it offers flexibility and excellent capability for the direct adaptive control of nonlinear chemical processes.