diff --git a/tests/test_roundtrip_interpolated.py b/tests/test_roundtrip_interpolated.py
new file mode 100644
index 0000000..13fd330
--- /dev/null
+++ b/tests/test_roundtrip_interpolated.py
@@ -0,0 +1,41 @@
+from src.codex32.codex32 import Codex32String
+
+
+# secret share from seed
+s = Codex32String.from_seed(bytes.fromhex("68f14219957131d21b615271058437e8"), "ms13k00ls")
+assert s.s == "ms13k00lsdrc5yxv4wycayxmp2fcstppharks8z0r84pf3uj"
+
+# derive 'a' via proposed BIP-85
+a = Codex32String.from_seed(bytes.fromhex("641be1cb12c97ede1c6bad8edf067760"), "ms13k00la")
+assert a.s == "ms13k00lavsd7rjcje9ldu8rt4k8d7pnhvppyrt5gpff9wwl"
+
+# derive 'c' via proposed BIP-85
+c = Codex32String.from_seed(bytes.fromhex("61b3c4052f7a31dc2b425c843a13c9b4"), "ms13k00lc")
+assert c.s == "ms13k00lcvxeugpf00gcac26ztjzr5y7fkjl7fx7nx7ykhkr"
+
+# derive next share via interpolation
+d = Codex32String.interpolate_at([s, a, c], "d")
+assert d.s == "ms13k00ldp4v5nw8lph96x47mjxzgwjexe44p32swkq99e0w"
+
+# now round-trip d share ('d' is derived via interpolation, NOT via 'from_seed')
+dd = Codex32String.from_seed(d.data, "ms13k00ld")
+# they are NOT equal after round-trip - seem we miss padding at interpolation level
+assert dd.s != d.s  # FAIL (should equal)
+
+# irrelevant
+# e = Codex32String.interpolate_at([s, a, c], "e")
+# assert e.s == "ms13k00lezuknydaaygk5u20zs4fm736vj909mdj6xqp8pc2"
+#
+# f = Codex32String.interpolate_at([s, a, c], "f")
+# assert f.s == "ms13k00lf0ehe53zsu6vrxcjjh9v7wzsa83mqfvku3fd8kem"
+
+# recover from shares, use 'd' without round-trip
+rec_s = Codex32String.interpolate_at([a, d, c], "s")
+# recover from shares, use 'd' after round-trip
+rec_ss = Codex32String.interpolate_at([a, dd, c], "s")
+
+print("     s:", s.data.hex())
+print(" rec_s:", rec_s.data.hex())
+print("rec_ss:", rec_ss.data.hex())
+assert s.data == rec_s.data
+assert s.data == rec_ss.data  # FAIL