We study the spectral properties of the first 14 observations of the rise to outburst phase of the X–ray transient J1550–564 .
Using both the PCA and HEXTE instruments , we find that the 3–200 keV spectra smoothly pass from a standard low/hard state to a very high state .
The classic high state is never encountered possibly indicating that it is not a phenomenon of the rise phase .
We find that the individual PCA spectra can be fitted adequately by a disk black body and a thermal Comptonization model which includes reflection .
Once the very high state is reached there is clear spectral curvature of the continuum which possibly indicates the presence of a composite thermal/non-thermal plasma .

Our detailed modelling of the reflection parameters shows a sharp increase in mean ionization at the onset of the transition between the low state and very high state .
There is a related variability in the reflected fraction but its exact value depends on the continuum model used .
The reflected fraction varies around values of \Omega / 2 \pi \sim 0.1 and is never consistent with \Omega / 2 \pi = 1 .
We can constrain the inner radius using relativistic smearing and while there are large uncertainties , the data are incompatible with a disk extending to the last stable orbit ( 6 R _ { G } ) in either state .

Since the system is on the rise to outburst , the disk instability models ( and observed increasing QPO frequency ) strongly imply that there is no standard inner disk at the time the low state spectrum is observed .
This is compatible with a truncated disk , filled by an X–ray hot , advection dominated accretion flow .
However , magnetic flares above the outbursting disk can also match the observed spectra once the effects of either outflow and/or strong photoionization of the surface of the disk are included .
We clearly see strong ionization of the reflector in the very high state .
This is probably from collisional ionization , as the disk surface temperature is \sim 0.7 keV .
This can strongly suppress reflection from the inner disk .