Cyclic liquefaction of soils with a clearly positive state parameter is a form of instability triggered by undrained cyclic loading, where instability is used in the context of continuum mechanics, i.e. a state of dsijdeij < 0. This paper investigates such behaviour in the frame of critical state soil mechanics (CSSM) and taking into account the influence of fines content and cyclic stress reversal. The concept of an equivalent state parameter was used in conjunction with instability stress ratio obtained from monotonic undrained tests to synthesis the cyclic tests results. Experimental results showed that instability stress ratio obtained from monotonic test at the same equivalent state parameter define the triggering of instability under cyclic loading irrespective of stress reversal. The influence of initial effective confining stresses and fines contents can be assessed from the instability stress ratio and the equivalent state parameter.