Structural transformation between a dense molecular fluid and a polymeric liquid of phosphorus that occurred at about 1 gigapascal and 1000 degrees C was investigated by in situ x-ray radiography. When the low-pressure fluid was compressed, dark and round objects appeared in the radiograph. X-ray diffraction measurements confirmed that these objects were the highpressure liquid. The drops grew and eventually filled the sample space. Decompressing caused the reverse process. The macroscopic phase separation supported the existence of a first-order phase transition between two stable disordered phases besides the liquid-gas transition. X-ray absorption measurements revealed that the change in density at the transition corresponds to about 40% of the density of the high-pressure liquid.