In previous study, we found that Mg(H₂PO₄)₂ instead of NH₄H₂PO₄ was available as a binder material for phosphate-bonded investments and possibly could be used to develop the phosphate-bonded investment without ammonia gas release. The purpose of the present study was to develop the experimental ammonia-free phosphate-bonded investments by investigating suitable refractories. Mg(H₂PO₄)₂⋅nH₂O and MgO were prepared as a binder. Cristobalite and quartz were selected as refractories. The powder ratio of MgO/Mg(H₂PO₄)₂⋅nH₂O was set constant at 1.2 according to our previous findings. Fundamental properties of dental investment such as strength, manipulation and expansion were evaluated. Using cristobalite as the refractory material, further investigations were performed. The refractory/binder ratio was definitely effective. The increase of this ratio led to low mold strength and large mold expansion. The present findings suggested that C5 was desirable for dental investment.