  Electron spin resonance experiment reveals that non-magnetic bond doping of
the spin-gap magnet (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ (abbreviated PHCC) results
in the formation of $S=1$ paramagnetic centers that dominate low-temperature
ESR response. We have followed evolution of this signal with doping impurity
content and have found that these centers concentration is quadratic over the
impurity content. We also observe coexistence of the ESR responses from these
local centers and from delocalized triplet excitations over certain temperature
range.
