Single crystals of Tb-Tm alloys with Tm contents of 12%, 40%, 55% and 65% were investigated by neutron diffractometry over the temperature range 4.2-300K. All these alloys order magnetically to a basal plane spiral below the Neel temperature. Below the Curie temperature the magnetic ordering of the Tb-12% Tm alloy is ferromagnetic with the moments in the basal plane, while the other alloys have a ferromagnetic component along the c axis and a spiral component in the basal plane below Tc. The Tb-55%Tm and the Tb-65%Tm alloys have an intermediate mixed phase, where the c axis component is modulated along the c axis (CAM structure). Tb-Tm alloys have an inhomogeneous phase, where the spins associated with the Tb ions lie in the basal plane, while the spins associated with the Tm ions order along the c axis.