Cellular eta decomposition in three Fe-26Ni-(2-3)Ti-Nb type steels was investigated. One of these steels was alloyed with molybdenum while the two others, differing in grain size, contained an addition of aluminium. In Ni26AITi3Nb and Ni26MoTi2Nb steels quenched from 1223 K in water, the first colonies of eta decomposition were observed after holding for 4 h at 923 K. A longer holding at this temperature resulted in growth of eta decomposition cells in the steel with molybdenum while in the steels with aluminium, a halting of the decomposition and the appearance of isolated plates of the eta phase was observed. In Ni26MoTi2Nb and Ni26Al2Ti3Nb steels, quenched in water and subsequently cold treated in liquid nitrogen, repeated heating in the temperature range 293-999.9 K at a rate of 2.5, 10 and 40 K min(-1) and cooling at a rate 320 K min(-1) favoured the cellular eta decomposition more markedly in the steel with molybdenum. On cooling to room temperature, austenite within the decomposition cells transformed into martensite. The tendency to cellular decomposition depended to a higher degree on the factors which stabilize austenite as well as on the presence of precipitates in grain boundaries, than on the grain size.