The biogenic Si concentration in a sediment can be determined as the non-detrital Si concentration by normalization of the total Si concentration with Al. This procedure is based upon the assumptions that (1) that Al exists predominantly in detrital, i.e. minerogenic, particles and (2) that biogenic Si (mainly diatom frustules) is the dominating non-detrital Si phase. This paper focuses on the reasons for the variations of the non-detrital Si concentration in these lake sediments. Data from three lakes are presented, representing three principally different cases regarding the Si concentration. The processes controlling the concentrations of detrital and non-detrital Si are discussed.