We show numerically that in the model of trimolecular reaction under external periodic force (the forced Brusselator) there exists the intermittent route to chaos. The time development of intermittent chaos and the method to distinquish intermittency from transients are studied. The large region of period 3 in the parameter space, discovered previously in the forced Brusselator, as well as smaller regions of periods 4, 5, 6 … etc., correspond to tangent bifurcations in one-dimensional mappings. Intermittency appears just before the start of every tangent bifurcation. Therefore, the period-doubling and the intermittent routes to chase are "twin" phenomena and they should be observable in many other systems described by nonlinear differential equations.