Abstract: Under a certain experimental condition, a-type of heavier rare earths chelates of chlorophosphonazo (III) can be converted to β-type. In continuous scanning spectrogram, it is shown that α-type is formed instantaneously. The α-peaks decline and the β-peak raises when the reaction is in process. when Yb:R = l:1 and pH = 3.2, we had determinated at constant temperature that the order of reaction is equal to 2 and 1 for nonbuffer system and buffer system (NaAc-HCl) while apparent activation energy is equal to 71.7 KJ/mol and 49.8KJ/mol, respectively. For many buffer systems of interest, it is clear that buffer medium of having nucleophilic groups can accelerate the converted process. In these buffer systems the observed formation rate of β-type chelates decreases with increasing of hydrogen ion concentration and increases with the buffer medium concentration. The experimental result tells us that two a-type molecules combine into a β-type molecule and liberate a proton at the same time.