During attempts to produce novel two-dimensional cocrystals by coadsorbing components in a binary mixture, the formation of a metastable form was observed in analogy to the phenomenon of additive-induced polymorph formation reported in three-dimensional crystallization. Mechanistic in-sights into this phenomenon were gained through the use of scanning tunneling microscopy and several adsorbate/additive combinations. One additive plays a critical role in forming a disordered assembly through a process that is primarily kinetic whereas another additive thermodynamically stabilized an intermediate form, resulting in interrupting a phase transformation to a more stable form. These additive effects elucidate one of the potential pathways to kinetically isolate a metastable polymorph formed during cocrystallization in three-dimensional crystallization.