Fluorescence spectroscopy and H-1 NMR were used to study linear poly(methylphenylsiloxane) (PMPS) in dilute solution (2 x 10(-4) to 4 x 10(-3) M) at temperatures below -50 degrees C. Several PMPS samples covering a range of number-average molecular weights ((1.9-93) x 10(3)) and of low polydispersity were employed. The temperature dependence of steady-state fluorescence results show two different trends, depending on chain length: (i) for the high molecular weight samples, the Stevens-Ban plot of excimer-monomer intensities levels off when reaching temperatures below -50 degrees C and yields an apparent rate constant for excimer formation (k(a)) which is independent of temperature; (ii) for the low molecular weight samples, the expected bell-shaped plot is observed, and k(a) increases with temperature in the whole range here studied (153-293 K), including also the temperatures below -50 degrees C. Likewise, light scattering at the incident radiation wavelength increases abruptly upon decreasing temperature below -50 degrees C for the high molecular weight samples. An anomalous behavior of high molecular weight samples is also observed in H-1 NMR spectra; the signals disappear in the baseline at temperatures below -50 degrees C, indicating a drastic loss of mobility in the millisecond time range. A conformational change coupled with interchain aggregation is argued to explain these results.