4-nitro phenol  is a slightly yellow, crystalline material, moderately toxic. 
It shows two polymorphs in the crystalline state. The alpha-form is colorless pillars, unstable at room temperature, and stable toward sunlight. The beta-form is yellow pillars, stable at room temperature, and gradually turns red upon irradiation of sunlight. Usually 4-nitrophenol exists as a mixture of these two forms.
In solution, 4-nitrophenol has a dissociation constant (pK) of 7.15 at 25 °C.