The Gensler and the Fujiwara processes for the Knoevenagel reaction of methylma-lonio acid and derivatives with aromatic aldehydes have been reinvestigated. The Gensler process has a wider range of application than the Fnjiwara prooeas and the yields are higher too. The cause is attributed to difference in mechanisms. It is believed that the Gender process involves the Knoevenagel or Rodionov mechanism whereas the Lapworth mechanism works with the Fujiwara process. The Gender process requires at least one free carbonyl group on the methylmalonio acid part and even so, it still does not react with α-cyanopropionio acid. Sterio factors are believed to be the cause.