diff --git a/fastq-demultiplexing/Python/fq_demux.py b/fastq-demultiplexing/Python/fq_demux.py
index f5a253ed088e3e6185a007d7784fdcd478a85df9..7fadcbd959ce1b568cb556e8894f40829c439d9a 100755
--- a/fastq-demultiplexing/Python/fq_demux.py
+++ b/fastq-demultiplexing/Python/fq_demux.py
@@ -76,8 +76,8 @@ def main():
     bc_start = args.barcode_start - 1
 
     def bc_prob_profile(seq, qual):
-        # return [(letter, 10 ** (-quality / 10))
-        return [(letter, 10 ** (-ord(quality) / 10))
+        # return [(letter, 10 ** ((33 - quality) / 10))
+        return [(letter, 10 ** ((33 - ord(quality)) / 10))
                 for letter, quality
                 in islice(zip(seq, qual), bc_start, bc_start + bc_len)]