Parallel Rosenbrock methods are developed for systems with stiff chemical reactions. Unlike classical Runge-Kutta methods, these linearly implicit schemes avoid the necessity to iterate at each time step. Parallelism across the method allows for the solution of the linear algebraic systems in essentially Backward Euler-like solves on concurrent processors. In addition to possessing excellent stability properties, these methods are computationally efficient while preserving positivity of the solutions. Numerical results confirm these characteristics when applied to problems involving stiff chemistry, and enzyme kinetics.