Single crystals of the quaternary crystalline solid solution system Rb x K 1-x Br y Cl 1-y have been grown by melt technique for three different compositions y = 0.25, 0.50 and 0.75, keeping x = 0.5 and X-ray diffraction studies have been carried out for each case independently, using spherically ground single crystals. The results indicate an enhancement of Debye-Waller B(total) factors for the compositions y = 0.25 and 0.75 whereas for the intermediate composition, y = 0.5, the factor B(total) is close to the end member value. This experimental investigation show that for this composition, the crystalline solid solution behaves like an end member crystal.