  Hall effect measurements were performed under pressure and magnetic field up
to 2.2 GPa and 16 T on a single crystal of UCoAl. At ambient pressure, the
system undergoes a first order metamagnetic transition at the critical field
B_m = 0.7 T from a paramagnetic ground state to a field-induced ferromagnetic
state. The Hall signal is linear at low field and shows a step-like anomaly at
the transition, with only little change of the Hall coefficient. The anomaly is
sharpest at the temperature of the critical end point T_0 = 12 K above which
the first order metamagnetic transition becomes a crossover. Under pressure B_m
increases and T_0 decreases. The step-like anomaly in the Hall effect
disappears at P_M ~= 1.3 GPa and the metamagnetic transition is not detected
above the quantum critical end point (QCEP) at P_Delta ~= 1.7 GPa, B_m ~= 7 T.
Using magnetization data, we analyse our Hall resistivity data at ambient
pressure in order to quantitatively account for both ordinary and anomalous
contributions to the Hall effect. Under pressure, a drastic change in the field
dependence of the Hall coefficient is found on crossing the QCEP. A possible
Fermi surface change at B_m remains an open question.
