A series of isostructural homo- and heterolanthanide coordination polymers of formula [Ln(dcpa)(HO)] with Ln = La-Gd have been obtained by reactions in water between the lightest lanthanide chlorides and the disodium salt of 4,5-dichlorophthalic acid (Hdcpa). They present particularly high thermal stability for coordination compounds (up to 400 °C). Their luminescent properties have been studied in detail. Interestingly an insensitivity to water coordination as well as a very strong effect of optical dilution is observed. Therefore, molecular alloys with very high lanthanum concentration have been prepared. Some of them present highly tunable and very intense luminescence. For example, to the best of our knowledge, [SmLa(dcpa)(HO)] presents one of the highest overall quantum yields measured so far for a Sm-based coordination compound ( = 9.2%), and [NdSmEuLa(dcpa)(HO)] is one of the brightest (12 Cd·m under 0.75 mW·cm UV flux) multiemissive visible and near-infrared lanthanide-coordination polymers reported to date.