The cloud-point temperatures (T-cl) of 1 wt % poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) (BnEmBn) and of 1 wt % poly(oxypropylene)-poly(oxyethylene)-poly(PnEmPn) were measured by detecting the sharp decrease in transmittance of the incident laser beam when phase separation occurs. The cloud-point temperatures were studied as a function of the length of both hydrophobic and hydrophilic blocks. A linear increase in the hydrophobic block length (B or P) leads to an exponential decrease in the cloud-point temperature. However, the middle hydrophilic block (E) shows only a weak positive effect; i.e., an exponential increase in the block length leads to a linear increase in the cloud-point temperature. On the basis of the available data, we can summarize our results in mathematical form and present a three-dimensional plot to predict the T-cl of BnEmBn (or PnEmPn) triblock copolymers. The middle E block has a stronger effect on PnEmPn than BnEmBn. It is also noted that one oxybutylene (B) unit has the effect equivalent to about 4.4 oxypropylene (P) units.