In-situ time-resolved small angle neutron scattering is applied to the guest exchange phenomenon in the co-crystals of syndiotactic polystyrene for the first time, exploiting the significant difference in coherent scattering length between two isotopologues, fully protonated and deuterated compounds, employed as originally-occupying and subsequently-occupying guests. The vapor-induced guest exchange process between protonated toluene and deuterated one is followed by monitoring the reflection due to crystalline lamellae, which confirms that the exchange of small guest molecules like toluene proceeds smoothly in the crystalline region. In such a case, the rate of exchange is much higher than the rate of crystalline complex formation from the amorphous state induced by exposure to a vapor of guest. (c) 2013 Elsevier Ltd. All rights reserved.