The gelation mechanism was investigated using in situ ultrasonic measurements. It was found that the gelation takes place by different mechanisms, depending upon quenching temperature. When iPS/trans-decalin solution was allowed to quench above the coexistence curve, the gelation was caused by crystallization. On the other hand, when it was allowed to quench below the coexistence curve, the gelation was caused mainly by liquid-liquid phase separation. Therefore, the junction point structure may be crystallite for the former gels, while for the latter ones, it may be a solvated structure. This conclusion was also supported by other measurements such as IR and NMR spectra, as well as by visual and tactual observations.