The solidification of random isotactic copolymers of propylene and 1-butene has been followed in real time by wide-angle X-ray scattering as a function of the rate of cooling the quiescent liquid. The experimental setup allowed simultaneous recording of cooling curves-sample temperature as a function of time-and X-ray patterns at high sampling rate of 20 Hz. This approach allowed establishing a correlation between cooling rate, temperature of crystallization/mesophase formation, and X-ray structure, which formerly has only been observed ex situ, after completion of structure formation during cooling and subsequent aging. It is quantitatively confirmed that addition of 1-butene co-units into the propylene chain allows mesophase formation on cooling the melt at distinctly lower rate than in case of the homopolymer. The experimental results are compiled into a continuous cooling transformation (CCT) diagram and compared with data obtained earlier on random copolymers of propylene with ethylene.