In this study, two phosphonium based ionic liquids (butyl triphenyl phosphonium chloride and hexyl triphenyl phosphonium bromide) have been synthesized using quarternisation process. The toxicities of these Ionic Liquids (ILs) are unknown and may be harmful to humans and the environment. Therefore, the toxicity assessment of these ILs was carried out according to the Organization for Economic Cooperation and Development (OECD) 203 guideline using female guppy fish (Poecilia reticulata). The median Lethal Concentrations (LC50) have been estimated for butyl triphenyl phosphonium chloride and for hexyl triphenyl phosphonium bromide to be 73.35 and 61.36 mg L-1 respectively. Both LC50 obtained can be identified as slightly toxic ILs based on Acute Toxicity Rating Scale by Fish and Wildlife Service (FWS). The findings from this study can be used for better design of phosphonium-based ILs with consideration of their aquatic toxicities. © 2013 Science Publication.